What is Digimon, the Franchise?
Digimon is a spinoff of Tamagotchi aimed at boys, focusing more on raising Monsters for battle rather than to care for them. It eventually grew into a multi-media franchise, I'll be focusing on the more popular franchises over the V-Pets.
Digimon Continuity
To preface this, I'll list off the different continuities of Digimon, I'll assume you have a working knowledge of how Pokémon works to save time.
- Digimon V-Pets
- Digmon Adventure V-Tamer 01 (Manga)
- Digimon Adventure (2 Anime series, 3 Canon Movies and 1 non-canon movie, and Digimon Adventure Tri, a series of movie sequels)
- Digimon Tamers (Anime, two movies that may or may not be canon)
- Digimon Frontier (Anime, one non-canon movie)
- Digimon Savers (Anime, one non-canon movie), called Digimon Data Squad in the West
- Digimon Xros Wars (2 Anime series), called Digimon Fusion in the west, has an alternate retelling in manga form.
- Digimon Universe: Applimon (Anime, and Manga)
- Digimon X-Evolution (a movie unrelated to the others)
- C'Mon Digimon (manga)
- Digimon Chronicle (manga)
- Digimon D-Cyber (manga)
- Digimon Next (manga)
- Digimon World (series of games)
- Digimon Story (a series of games that expanded upon Digimon World 3's formula, I think, this has multiple continuities)
Naturally, I won't cover
every continuity, since then I'd be here all night, and most of them have similarities anyway.
Pokémon World vs. The Digital World and More Digimon Continuity
In Digimon, there is a “Digital World”, a parallel dimension connected to the “Real World” via computer networks (it's weird, bear with me). There are also multiple unconnected universes: for example, there are six Anime continuities and multiple manga and game continuities, though this is not really any different from Pokémon having the game universe, the anime universe, the manga universes and Mystery Dungeon and other spinoff game universes.
The Digital World is separate from the Real World, and in some continuities time passes much faster in the DW than in the Real World. In Pokémon, the Pokémon world is the real world, and Pokémon are theorized to come from space.
Prior to Digimon Xros Wars: The Young Hunters who Leapt through Time, there was only one major connection between anime continuities: The protagonist of a series of Japan Exclusive Wonderswan games (Digimon Adventure: Anode Tamer & Cathode Tamer and it's sequels), eventually crossed dimensions from Digimon Adventure's continuity into Digimon Tamers'... somehow. In the dub this was never explained, but still hinted at. In Young Hunters, all the previous anime Main Characters reappeared to help the latest protagonists defeat the Big Bad of that series. Why? Anniversary tie-in.
Pokémon Trainers vs. Digimon Tamers
There are characters, usually but not always children, called “Digimon Tamers”. Depending on the version, they can have just one “Digimon Partner” (Digimon Adventure V-Tamer 01, Digimon Adventure and it's sequels, Digimon Tamers, Digimon Savers and Digimon World) or multiple (Digimon Xros Wars, the extreme majority of the games). In one series, the characters don't have Digimon Partners, they instead become Digimon, this is Digimon Frontier. In the Digimon Adventure anime, “Digimon Tamers” are called “Chosen Children (JP)/Digidestined (Dub)”.
Digimon the species vs. Pokémon the species
In most Digimon universes, Digimon are usually unknown, or a mystery (much moreso than they are a mystery in Pokémon).
Digimon are always sapient creatures that can speak human languages, while certain Pokémon in certain continuities can learn to speak English (the anime Meowth) or can communicate telepathically (Mewtwo; Lucario; Zoroark; Arceus), they are the exception, not the norm. Pokémon are usually treated like animals, while they can understand human commands, this is little different to training a Dog.
Certain games, like Digimon Story: Cyber Sleuth Hacker's Memory, actually bring up the implications of people treating Digimon as tools.
How Tamers acquire Digimon
In the continuities where a Tamer has only one partner, it's more of a soul-link bond kind of thing (hey, like Ash-Greninja!). They have one Digimon who will generally reflect their personality.
In the continuities where a Tamer has multiple Digimon... well, there are actually three methods:
In Digimon Xros Wars, the characters recruit armies of Digimon to fight with (with one main), they then fuse the Digimon together (DigiXros/DigiFuse) to achieve higher power. This is a case of the Tamer and Digimon having a common goal of defeating the Big Bad/fighting for the Big Bad, or the Digimon settling a debt with the army.
In Digimon Story: Cyber Sleuth, there is a character (who is not the main character), who has two Digimon partners who she treats like the Digimon from previous series. In Digimon World: Next 0rder, most Digimon Tamers have one partner, except the main character who has two for some reason and is considered special because of it. There is also a character in V-Tamer 01 who also has two Digimon. When a Tamer has two Digimon, it's almost always Agumon and Gabumon (who are, or have, evolved into WarGreymon and MetalGarurumon), who then fuse into Omegamon (Omnimon in the Dub). As the player character, you are given the freedom to deviate from this mold. In the Digimon Adventure movie “Hurricane Touchdown!” (part three of Digimon: the Movie), there is Wallace (Willis in the dub), who has Terriermon and Lopmon.
In the Digimon Story games, it plays a little bit more like Pokémon (you collect teams of Digimon to use in battles). Even there, it is different to Pokémon, when facing a Digimon, you scan data on them, when you have scanned enough data, you can generate a new instance of that Digimon.
Evolution
Like Pokémon, Digimon evolve (well, to get pedantic, both undergo metamorphosis, but that's splitting hairs). They both also hatch from Eggs (though Pokémon do so in a more natural way... or so we think... Digimon eggs are usually just reincarnated Digimon.
In the Dub, evolution is called Digivolution, presumably to differentiate it from Pokémon, also they added "Digi" to everything!
There are a few key differences.
Firstly, Digimon have twice as many evolution stages as a Pokémon can; Pokémon can only have a maximum of three not including the Egg stage, four with Mega Evolution; a Digimon has six stages not including Egg stage, or seven if you count the rare Super Ultimate stage)
Digimon Evolution Stages (Sub/Dub): Digi-Egg > Baby I/Fresh > Baby II/In-Training > Child/Rookie > Adult/Champion > Perfect/Ultimate > Ultimate/Mega > Super Ultimate/Ultra
When left alone, Digimon will evolve over time; however, the process can be sped up by having a Digimon Tamer (the method of this varies depending on whether it's the anime or games). If a Digimon evolves naturally, the evolution is permanent, just like Pokémon; however, if they evolve by means of their Digimon Tamer, the evolution is temporary. This is just like Pokémon's Mega Evolution concept, and has been a staple of the series since Digimon Adventure.
Digimon can also fuse, this is called "Jogress Evolution" (DNA Digivolution in the Dub) or "DigiXros" (DigiFusion in the Dub). DigiXros is exclusive to the Xros Wars series, and is more like a MegaZord kind of thing. (As an aside, "Jogress" is a portmanteau of "Joint Progress").
Generally, there is no true way a Digimon will evolve, their evolution trees are huge webs of cross pollination. As an example, an Agumon can evolve into Greymon, GeoGreymon, Veedramon, Sukamon, Centarumon, Leomon, Monochromon and a few others. Veemon can also evolve into Veedramon, and other Digimon also have similar crossovers.
Most Digimon have 'official' lines, for Agumon it's: Botamon > Koromon > Agumon > Greymon > MetalGreymon > WarGreymon > Omegamon and Botamon > Koromon > Agumon > GeoGreymon > RizeGreymon > ShineGreymon > ShineGreymon: Burst Mode, but these aren't necessarily what all Agumon will evolve into.
When Digimon die, they reincarnate into a new Digimon, starting from an egg; however, this only applies in the Digital World, if a Digimon dies in the real world, they're gone for good (except as ghosts). This is a plot point multiple times.
Gods of Digimon and Pokémon
Both series have Gods, go figure. In Pokémon, the top god is Arceus, but Arceus might just be an uber-powerful Pokémon. Digimon's Top God, Yggdrasil, is the "Main Server" of the Digital World, it's not really a Digimon, in multiple continuities, Yggdrasil becomes faulty and is replaced by "Homeostasis", though the being didn't get a name until the Xros Wars manga, it appeared as early as Digimon Adventure.
Digimon also has other gods based on the gods of other religions (example: the "Olympos XII" guess who they're based on). But they're just really, really powerful Digimon. There's also the "Seven Great Demon Lords" (Seven Deadly Digimon in the dub), who are... well, the seven deadly sins.
Digimon really, really likes referencing mythology in an overt way, while Pokémon is more covert with it's mythological references.
Types
This is a big difference between Digimon and Pokémon, in Pokémon there are 18 Types (Normal, Grass, Fire, Water, Electric, Fighting, Flying, Poison, Psychic, Ghost, Bug, Rock, Ground, Ice, Dragon, Steel, Dark and Fairy). In Digimon, there are six main types, called Attributes: Vaccine, Data, Virus, Free, Variable and Unknown. The first three Attributes have a Rock-Paper-Scissors relationship Vaccine > Virus > Data > Vaccine; Free and Variable are always tied with the main ones, and Unknown is always dominant.
In the games, there are other sub-types: Fire, Water, Plant, Wind, Electric, Rock, Metal, Holy, Evil, Neutral and "Filth", they have their own RPS relationships (Water > Fire > Plant, and Electric > Wind > Rock) that are unconnected to the Attributes.
A Digimon will be one Attribute and one Type, unlike Pokémon who can have one or two types.
Some Digimon have abilities that grant them advantages over other families of Digimon, for example WarGreymon's claws are especially effective against Draconic Digimon (Digimon with 'dramon' in their name); this only really applies to the Anime.
Pokémon Vs. Digimon
In America, Pokémon and Digimon were localised at about the same time, with Digimon probably being localised as an attempt to cash in on the "Mons" craze. In Japan, Pokémon Red & Green released in February 1996, Tamagotchi started in November of the same year, and Digimon was spawned from Tamagotchi when Bandai wanted to create something more masculine to appeal to Boys. While there is wriggle-room to argue that Tamagotchi (and therefore Digimon) were created in response to Pokémon, it's unlikely.
This is an interesting little fact that is up to opinion, but the general consensus of fans of both Pokémon and Digimon is that Pokémon has the better games, while Digimon has the better anime.