A forum or a community forum is a common place where come to discuss, share knowledge, ask questions, get feedback, and interact with each other usually about a specific area of interest. Forums help users of a product, or supporters of a company, get more intimate with not just the product but with the people who are there for the same reason.
Forums can be considered community platforms but are more encouraging towards longer posts and interactions as compared to other community platforms. A forum typically consists of multiple categories, each serving a specific purpose of discussion. Depending on whatever category it may be, a member or a user can make a post and initiate a dialogue where interested folks can continue the conversation by replying to that post. Most, if not all, forums also have an upvote feature that allows people to show their support or liking towards that post.
For example - Thunkable has a Forum based community on Discourse.