A bulletin board, also known as a message board or internet forum, refers to online discussion websites that internet users may join. Most of these websites require users to register an account before posting, but a small minority of boards do permit anonymous posting.

Most people attribute “bulletin board” and “bulletin board system” (BBS) to early internet discussion zones, while “message board” or “internet forum” are colloquially used to describe modern bulletin boards. While Japanese users say “electronic bulletin board” (電子掲示板, denshi kejiban), English users would arbitrarily split them into “textboard” and “imageboard”, then call an image-sharing website a “booru”.

In short, we've end up with several terms that essentially describe the same thing which isn't ideal. To avoid overcomplicating things, I'll be using “bulletin board” and “internet forum” to describe these websites.

There's been a decline in niche forums, especially since a “megaforum” like Reddit kills the need to spread out since you can already discuss everything under one place. In addition, there's been a bigger focus on preservation (e.g. archives, backups, etc.) these days, which means that less people will experience the relief of not being able to read your embarrassing old posts after the website has disappeared.

• Internet forum on Wikipedia