Linux HOWTOs are documents which describe in detail a certain aspect of configuring or using Linux. For example, there is the Installation HOWTO, which gives instructions on installing Linux, and the Mail HOWTO, which describes how to set up and configure mail under Linux. Other examples include the NET-3 HOWTO and the Printing HOWTO.
HOWTOs are comprehensive docs - much like an FAQ but generally not in question-and-answer format. However, many HOWTOs contain an FAQ section at the end.
There are several HOWTO formats available: plain text, PostScript, DVI, and HTML.
In addition to the HOWTOs, there are a multitude of mini-HOWTOs on short, specific subjects.