What is forum software?
To us, forum software is a group of people interested in a common topic who are willing to type paragraphs to each other on a web page.
Yes, there is Twitter for very short form, there is web chat and IRC for real time with limited persistence, there is Tumblr for micro-blogging, and there are blogs for, well, blogging. But we believe forums are a fundamental building block of web community.
Here's a screenshot of a forum as it existed in the year 2000, alongside a screenshot of the same forum as it exists today.