With tables you use line breaks and vertical lines to separate them. For example, if your code looks like this...
[TABLE="head"]Top-Left|Top-right
Bottom-Left|Bottom-Right[/TABLE]
...it would make a table that looks like this:
Top-Left | Top-right |
---|
Bottom-Left | Bottom-Right |
If you don't include the ="head", it would look like this:
Top-Left | Top-right |
Bottom-Left | Bottom-Right |
Just keep in mind that other hosts, such as phpBB, sometimes use the HTML way and use [tr] and [td] (not sure if they use [th]).