A SSD (Solid-state drive) is a data storage unit that unlike your standard hard disks doesn't have any moving mechanical components. As a result, it's cooler, more silent and responds/works faster than the HDD. The smartest thing to do with your SSD is to store your operating system on it. This will make your computer feel brand new if you have been struggling with a crap hard drive. My PC boots at around 15 seconds. It's also great for load heavy games such as Skyrim.
The downside is, obvioulsy, the price. It's many times more expensive per unit of storage compared to a HDD. I've also heard some SSD's have a tendency to be unstable and to crash, but this is not something I have experienced myself.
Is it worth the cost? Yeah, I would say so. Quality comes at a price, and a good SSD responds incredibly quick. You won't believe how fast your PC boots with this thing.