In general terms, documentation is any communicable material (such as text, video, audio, etc., or combinations thereof) used to explain some attributes of an object, system or procedure. It is often used to mean engineering or software documentation, which is usually paper books or computer readable files (such as HTML pages) that describe the structure and components, or on the other hand, operation, of a system/product.
A professional whose field and work is more or less exclusively to write documentation is called a documenter. Normally, documenters are trained or have a background in technical writing, along with some knowledge of the subject(s) they are documenting. Often, though, some part or all of the documentation process is done by the engineers responsible for the system/product to be documented.
By engineers, perhaps among software engineers in particular, documentation is often referred to as the "boring side" of engineering, or considered a necessary evil. This is largely unavoidable since most engineers prefer building things to documenting them, and being implicit experts in what they have built, they may have little motivation in documenting their creations so that others may understand them.
Common types of computer hardware/software documentation include online help, FAQs, HowTos, and user guides. The term RTFM is often used in regard to such documentation, especially to computer hardware and software user guides.
In some European countries, documentation in an academic context is an obsolete term for the field of study that is now known as library science or information science.