SSD = Solid State Drive. Once you have one, you'll wonder how you lived without it. I used to be against the prospect, stating all the usual excuses, they're new tech, they're not reliable, etc.
But I have to tell you, when my boss told me to get one for my workstation at work, I was like "Hey, "free" SSD to try... " so I did, and I'm so very glad I did.
I have a 120GB SSD in both my work PC here and my laptop at home, and it speeds things up incredibly. The 120GB drive is still big enough to install Windows, and at home, a few games (Minecraft, WoW, Rift for a time, etc) and it greatly speeds up access times to frequently used programs that you install to it, like Firefox, IE, Chrome, Office, etc.
To wit, my desktop at home has two Western Digital Raptor 150GB's in RAID 0. They're pretty fast. A single 120GB SSD smokes them when it comes to booting windows, loading Office, and even opening Firefox. Games I can't really test at work on this PC, but the laptop system is amazing, and it's only a 2.5 GHz dual core.
Hopefully that helps a bit, I know I gushed over SSDs as a whole, there are some bad ones out there, but the ones I link are fine.