In general terms, documentation is any communicable material (such as text, video, audio, etc., or combination thereof) used to explain some attributes of an object or system. It is often used to mean software documentation, which is usually paper books or readable computer files (such as HTML pages) that describe how to operate computer programs.
A person who writes documentation is a documenter. Normally, documenters are trained or have a background in technical writing, along with some knowledge of the subject they are documenting.
Documentation is often referred to as the "boring side" of engineering, or considered a necessary evil. This is largely unavoidable since most engineers prefer to build things instead of to document them, and being implicit experts in what they have built, they have little motivation in documenting their creations so that others may understand them.
The term RTFM is often used in regard to documentation.