Topics are called topics because the original post and its replies are meant to contain just one topic of discussion. On many forum boards, introducing "off-topic" information is considered rude. The topic's original poster (starter) usually defines what is and isn't "on topic."
On some forums, the administrator has given the topic's original poster (the member who made the first post) elevated permissions within the topics they start, making them an "owner" of the topic.
Polls and calendar events can be tied to topics, and topics can be made sticky or locked. Topics are always posted within one and only one board, and are subject to various forms of moderation.
By default the number of topics is limited to 16,777,216.