A contribution is on-topic if it is within the bounds of the current discussion and off-topic if not. The terms are normally used in the context of mailing lists, discussion groups, discussion forums, bulletin boards, newsgroups, and wikis.
It is a common courtesy to mark a new off-topic posting or email by beginning it with "OT" - for example in a forum discussing the Linux operating system someone might post: "OT: Wow, did you feel that earthquake?".
The term "off topic" is sometimes used to indicate a discussion venue where anything other than matters specifically addressed by the website in question is addressed. For example, an off topic category on an automobile related discussion forum may include talk about video games (which presumably have nothing to do with automobiles). There are a variety of sites dedicated entirely to random discussion that have nothing to do with anything.
Posting off-topic messages to deliberately aggravate other members is referred to as "trolling", and is common in some forums.