BBCode is short for Bulletin Board Code. It is used as a way for formatting posts made on message boards, blogs and more. It is similar to HTML in the sense that in BBCode one does also use tags to format something specific (contained within the tag). In BBCode tags are indicated by rectangular brackets surrounding a keyword, which is in turn transformed into HTML before being delivered to a web browser.
The basic BBCode tags are often very similar across many different forums, but there are some variants in existence as well. Sometimes BBCode tags has to be in specific cases (i.e. [b]bold text[/b] will work, while [B]bold tex[/B] will not). It is also very different as to which of the more unsafe and/or complex tags that are supported. For instance you cannot always expect the [img] image tag to be supported as allowing posters unlimited power to post any picture they like could have some pretty nasty effects.