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Raptor is a brand name for a fast hard-drive made by the Western Digital company. SSD stands for Solid State Drive.

Raptors, like all conventional (non-SSD) hard-drives, store their information on magnetic disks that spin inside them. The faster they spin the better, as it means information can be read from them and written to them quicker. The benefit of that is you don't have to wait so long for programs (including Windows itself) to load up and start, and anything you want your computer to do that requires accessing the hard-drive will be done quicker. Western Digital Raptors (or Velociraptors as they are also known) are the fastest, affordable, conventional hard-drives readily available to consumers.

SSD's are hard-drives that have no moving parts, using instead large amounts of what is called 'flash' memory, which is similar to the kind of memory that's used in removable SD Cards and the USB Flash Drives or 'memory sticks' that you might be familiar with. SSD's are much faster than even the fastest conventional disk-based hard-drive, and much smaller, quieter and cooler too. They're more expensive though, and more limited in the amount of data they can store. SSD's, or something similar to them, will probably replace conventional hard-drives altogether once their storage capacities go up and their prices come down.

At the moment, SSD's are now generally considered to be affordable enough, and to have a high enough storage capacity, to be a good substitute for the conventional hard-drives that you would usually put your operating system and other often-used programs on. A lot of people are still using a combination of SSD's and conventional hard-drives though, in order to still store larger amounts of data, that they don't access as often, on the higher capacity conventional drives.

Hope that makes sense. Getting an SSD for your operating system and often-used programs is something worth considering, but not absolutely essential (though if you do get one you'll probably never go back to conventional hard-drives).