Laptop Memory (commonly referred to as "RAM") is a form of data storage that allows data to be read and written in about the same amount of time regardless of the order it is accessed in. This makes it much faster than hard disk memory or optical discs, where read and write time depends on the data location. RAM is volatile memory - in other words any data on it is lost should the power be removed.
RAM is used to hold information that is constantly in use, as it is much faster to access it than from a hard drive. One common misconception as that adding RAM will increase the speed of your laptop, but this is only the case if you are close to using all your RAM. Make sure you can upgrade your laptop's memory first - many laptops do not have a spare SO-DIMM slot for memory expansion.