a back button could be anything from a form button, to a link and even an image...etc...etc.. etc...
all you really need to do is use the javascript history object, and join it with an onclick event...
so for a link you could do it like this:
Code:
<a href="url.com" onclick="javascript:history.go( -1 );return true;">[back]</a>
I used that code a long time ago, not sure on how standards compliant it is...but now that you see how easy it is, you could
always do a little research on the topic.
(java script should be one word, the forum separates it)

good luck.
oopss..forgot, can't have a url, it defeats the purpose of the back button...
so like the next post has it... use # in place of url.