A style is overview colour, table properties, CSS etc of the forum. Templates are where you can edit every apperance of anything on the forum (and I mean everything)
A replacement is a great feature for a 'Find and Replace' on your forum. Say if you have certain code that you want to use over and over again, you can add a replacement to say:
Find this code
Code:
<b>something</b><br />
and turn into