A forum is a format for discussions (such as opinions) and Q&A (such as how-to) questions. 

Discussion threads advocate an opinion or point of view,  or start a discussion of a subject. Q&A threads are used to solicit answers or opinions for a question and aim (through the "I have this question too" tool) to build consensus. Q&A posts are answered using replies, suggested answers, and verified answers.

A forum has 2 features:

Suggested answer/verified answer. The author of a reply can click Suggest as answer, which turns the thread header yellow. If another user clicks "This helps me," the answer (depending on thresholds determined in Administration) becomes a verified answer and the question header turns green. (However the answered status can be rejected.)"I have this question too." This feature on a forum thread question has a counter which increases whenever someone checks the "I have this question too" check box; checking this check box increases a counter displaying the number of people who've also had the question.

What is a forum thread?

A forum thread is a conversation. The question (or discussion statement) begins the conversation, and the replies to the thread starters are the responses.

A forum is based on the community archetype. In discussion threads, one member posts a topic, and other members respond. In Q&A threads, a member poses a question, and then works with other group members who answer the question. 

Participants can indicate the relevance of the question by clicking "I have this question too" or "This helped me." Discussion and Q&A forums work differently, but they both increase community engagement.