Default is "threaded", where replys are put after the post they are replying to. This version is indented, so replies to the same post are on the same level. Also with this version, you only see one post per page, and the indented list is only headers.

The other option is "linear", which shows the entire posts, in chronological order. I prefer that method, but it can sometimes be confusing when people reply to a post without providing context.