SSD hard drives utilize flash based memory to operate, similar to what you might find in a USB thumb drive. Installing one of these in your laptop reduces the overall weight significantly, saving much needed energy for that next great presentation.
Many SSD hard drives come in a which is ideal for laptops. For a desktop PC you will need a bay adapter though some other form factors are now available. Capacity wise there is a wide variety available though 256gb drive is a popular option.