FAQ is an abbreviation for Frequently Asked Questions. It is sometimes pronounced as an acronym, fack, and sometimes as the initialism F.A.Q. (Since it originated in the text medium of USENET, there is no agreed-upon pronunciation.)
The FAQ is an Internet textual tradition originating from periodic summary messages posted to USENET newsgroups in an attempt to reduce the continual reposting of the same basic questions. On USENET, posting questions which are covered in a group's FAQ is often considered inappropriate, as it shows that the poster has not done the expected background reading before asking others to provide answers. Some groups may have multiple FAQs on related topics, or even two or more competing FAQs explaining a topic from different points of view.
A text consisting of questions and their answers is often called a FAQ regardless of whether the questions are actually frequently asked (if asked at all). This is done to capitalize on the fact that the concept of a FAQ has become fairly familiar online.
The term "FAQ", and the idea behind it, has spread offline as well, even to areas not related to the Net at all. Even bottles of bicycle chain lubricant have been marketed with accompanying leaflets titled as a "FAQ".
There are thousands of FAQs available on many subjects. Several sites catalog them and provide search capabilities -- for example, the Internet FAQ Consortium.
Wikipedia has its own Wikipedia FAQ collection.