SSD or HDD?

One is a basically like a big USB stick that you put in side your computer and the other is a regular hard disk drive. The big usb stick doesn't have any moving parts, that makes it quiet, energy efficient, and extremely fast. It uses a hard disk interface (not usb) which means you can treat it just like a regular drive. RAID, partitioning, spanning all works just like it was a disk.

The down side being that its very pricey. Only certain vendors (like Intel) make good ones and some brands/models have strange issues. Capacity is small compared to disks. No one is sure how reliable they are compared to a regular hard disc drive.

This is a giant oversimplification but it should give you an idea. Please don't flame me endlessly for massive oversimplification. If you spend a great deal of money you will never make a better upgrade to your system. If you get a cheap one, you will hate the thing.