RAM's purpose is mainly storing loaded programs and the data they use, because it's faster to load and access information off RAM than other disk drives. Processors can act on it quickly, making things like program start-ups happen much faster. It is also used for temporarily storing data that might need to be accessed quickly, for example, when somebody edits a document, the changes to the text and formatting are stored on RAM until the user actually saves it on the hard drive.
RAM is volatile, meaning it isn't a good idea to use it as a long-term solution for storage. Everything is lost if the RAM chips aren't getting any power. So once a computer is shut down, all the data stored or available previously through RAM would be lost.
Random Access Memory is the opposite of sequential access memory, meaning you can access any part of it directly. With sequential access memory, you can only access data in the order in which it's stored, for example, if you have to listen to song number 4 on a cassette tape, you have to listen to everything that comes before it. RAM however, will allow you to directly access any song you would want to listen to, making access far easier and quicker.