SSD stands for Solid State Drive, and it uses flash memory (the same kind of memory used in a USB flash drive) to store data rather than the magnetic disk that a normal hard drive uses. They have no moving parts, are less prone to mechanical failure, use less energy, and can boot up your computer and load programs at over twice the speed that a normal 7200 RPM HDD can. The downside to SSDs is that they are much more expensive than HDDs, and its such new technology that's advancing so fast that what you could buy today could be outdated within a couple weeks.
TL;DR: SSDs are faster more reliable hard drives, but much more expensive to