SSDs are Solid State Drives – basically a hard drive made up of non-volatile memory with no moving parts. They run faster, cooler and extend battery life on laptops. I replaced the standard 500gb drive on my three-year old MacBook Pro with a new 500gb SSD and the boot time was reduced to a fraction of what it was before.
Since I’m a nerd, I clocked the boot time before replacing the drive, and it took 1 minute and 34 seconds to get to a login prompt. After the SSD was in place that time dropped to 19.5 seconds. In general the system is much faster and more pleasant to use.