BBCode is similar to the tags that you can use in forums to add special functions to your posts/reviews/etc. Here are the bbcode that Baka-Updates :: Manga supports.
[c]Copyright Symbol
[r]Registered Symbol
[tm]Trademark Symbol
[space]Hard Space
[img]Image url[/img]Image
[url]url[/url]Clickable link
[url=Url]Title[/url]Clickable link with title
[size=Size]Text[/size]Change size of text
[color=Color]Text[/color]Change color of text
[quote]Text[/quote]Set text as quote
[spoiler]Text[/spoiler]Set text as spoiler
[m]Manga[/m]Autolink manga to database
[a]Author[/a]Autolink author to database
[g]Group[/g]Autolink group to database
[font=Font]Text[/font]Change font of text
[list]List Items[/list]Create a list
[*]List Item[/*]Add item to list
And a special note, the parser attempts to convert bare links into clickable links