This wiki explains the basis of BBCode and has info for all (if not most) tags.
BBCode basics
[b] and [bold]
[i] and [italics]
[u] and [underline]
[s] and [strike]
[url] and [link]
[img] and [image]
[title] and [t1] - [t6]
[ol], [ul], [number], and [bullet]
[align]
[youtube]
[size]
[color]
[smallcaps]
[font]