Chords are sets of harmonically related notes, meaning pitches sounding at the same time. Go over to a piano and play C E G at the same time. That is a chord. Tertian chords (chords built thirds apart) are essential in tonal harmony. The triad is the most common chord. For example, C E G. This is a C major triad in root position.
Chords don't have to be tertian. For example, C D F# G# is also a chord. Doesn't sound that nice though, as it contains two tritones. So tl;dr version, chords are different pitches sounding at the same time.
Mike pretty much explained the tritone. It is an augmented 4th or a diminished 5th, meaning that the tones are six semitones apart. C to F#, A to Eb, Bb to E... these are all six semitones apart, making them tritones. They are very dissonant. This interval is used for most European emergency vehicle sirens for this reason, as it grabs people's attention due to it's crunchiness.
Brad, music is bro.