Random-access memory (usually known by its acronym, RAM) is a type of data store used in computers that allows the stored data to be accessed in any order — that is, at random, not just in sequence.
In contrast, other types of memory devices (such as magnetic tapes, disks, and drums) can access data on the storage medium only in a predetermined order due to constraints in their mechanical design.
Generally, RAM in a computer is considered main memory or primary storage: the working area used for loading, displaying and manipulating applications and data.
This type of RAM is usually in the form of integrated circuits (ICs). These are commonly called memory sticks or RAM sticks because they are manufactured as small circuit boards with plastic packaging and are about the size of a few sticks of gum.
Most personal computers have slots for adding and replacing memory sticks.
Most RAM can be both written to and read from, so "RAM" is often used interchangeably with "read-write memory." In this sense, RAM is the "opposite" of ROM, but in a more true sense, of sequential access memory.
The Wikipedia entry for RAM can be found here
In the video below, MvTCracker and Tigerdirect.com shows you how to install memory into your computer with some nifty advices as well.