A forum is a public discussion. It can refer to a meeting, a meeting house or any conversation that is available publicly.
There were forums — large public gathering places — in Ancient Rome; they can still be found across the globe. Traditionally, newspapers were thought of as forums for public discussions but more and more forums are virtual — discussion boards found online. Communities, however, still hold their own forums, or meetings, usually to discuss local issues. When you see the word forum, think public conversation.