A Solid-State Drive (or SSD) is a storage device that, unlike a standard harddrive, stores data on persistent flash memory. This enables mush faster seek and read rates, as well as better shock resistance than a standard HDD. Solid-State drives are still more expensive per gigabyte than HDDs, but prices continue to drop.
When the Operating System is installed on a SSD, boot times can be improved to the point of taking a single digit number of seconds to fully boot. This is often the second most recommended upgrade to speed up a computer behind installing more RAM, due to the cost involved.
Because SSDs tend to have a smaller capacity, there may not be room for the OS and all the games that you have. Thus, it is recommended that you only install performance intensive games on the SSD and install the others on a separate HDD. There exist programs that aid in moving programs between storage devices, like Steam Mover.
 Purchasing a SDD
 Differences from a HDD
There are some important differences between a SDD and a HDD. The first and most obvious one is performance. Due to a lack of mechanical parts, a SSD has much lower start up times, read times, noise. SSDs also have a much higher data transfer rate. Fragmentation causes no performance hit on a SSD, and on most Operating Systems defrag is disabled for SSDs.
In fact, there are only three major ways in which HDDs are better than SSDs. Most normal hard drives cost about 5 cents per gigabyte, while most SSDs cost about 65 cents per gigabyte. Not only that, but the majority of hard drives have a capacity of over 1.5 terabytes, while there are very few SSDs that are over 128 gigabytes. Also, the flash memory in a SDD has a limited numbers of times it can be written over, meaning that earlier SSD drives can wear out much faster than a HDD. This is becoming less of an issue, as more recent drives spread the data around in an effort to not wear out the flash memory as fast.