BB Code or Bulletin Board Code is a lightweight markup language used to format data. The available tags are usually indicated by square brackets ([]) surrounding a keyword, and are parsed before being translated into a markup language that web browsers understand. Examples:

[b]Bold[b] = Bold Text
[link=http://www.google.co.uk]Google[/link] = Google

BBCode was devised to provide a safer, easier and more limited way of allowing users to format their messages.

Text ©Wikipedia

Used on the following:

Category/product descriptions
New page text
Payment method information
Cart offline text

BB Code is simply converted to the correct XHTML markup using classes. To style these, edit the main styleheet and look for the BB Code attributes at the bottom of the file. stylesheet.css For example if you wanted to make all code that appears between [b][/b] tags blue, find: .bbBold {
  font-weight: bold;
}
And change to: .bbBold {
  font-weight: bold;
  color: blue;
}

BB Code can be uppercase or lowercase, but not a combination of both. The following are ok: [b]Bold[/b]
[B]Bold[/B] The following are NOT ok: [List]Bold[/LiST]
[dEL]Bold[/Del]

If you are using the BBcode tags for YouTube videos, you can alter the parameters in the 'control/defined.inc.php' file.

Look for the 'YOU_TUBE_EMBED_CODE' directive. Use {CODE} where the YouTube code should appear.

Note that using iframe data may break the code validation.

If you are using the BBcode tags for Vimeo videos, you can alter the parameters in the 'control/defined.inc.php' file.

Look for the 'VIMEO_EMBED_CODE' directive. Use {ID} where the Vimeo video ID should appear.

Note that using iframe data may break the code validation.

If you are using the BBcode tags for the soundcloud player, you can alter the parameters in the 'control/defined.inc.php' file.

Look for the 'SOUNDCLOUD_EMBED_CODE' directive. Use {URL} where the soundcloud url should appear.

If you are using the BBcode tags to load an mp3 file, you can alter the appearance in the 'control/defined.inc.php' file.

Look for the 'MP3_EMBED_CODE' directive. Use {MP3} where the mp3 path should load. Additional flashvars parameters as follows:

autoplay = 1 to auto-play
loadingcolor = The colour of loading bar
buttoncolor = The colour of the buttons
slidercolor = The colour of the player bar
byteslimit = If it is a mp3 streaming, the stream will restart at the bytes limit, for prevent overload.

More information here

BB Code is enabled/disabled globally in the settings.