An SSD, or solid state drive, is the single greatest upgrade you can give your computer. Unlike a conventional platter-based hard drive, an SSD has no moving parts. The makes for better reliability and better battery life for a laptop.
The greatest benefit is speed. On an older model laptop with a regular hard drive, it takes about ninety seconds to boot to the Desktop. With an SDD, this is cut to about twenty seconds. Opening applications is also faster. Now those icons in your Dock wont bounce a dozen times before your app opens. Saving and opening files is also faster. Simply put, an SSD will make your computer run faster than it did when it was new, and will extend the life of an older computer. Older hard drives can slow down as they get older. This is not the case with an SSD which will keep operating at the same blazing speed. And with no moving parts, the SSD is more reliable and better suited to the harsher environment of a laptop computer.