Okay, well
yippy is spot on for most of it but I think there are some very slight errors.
The Extreme can handle fifty wireless users whilst the Express can only handle ten.
Importantly the Extreme has a second ethernet port to hardwire other components (computers) to it whereas the Express has only a single ethernet port.
Technically, the two have the same broadcasting powers meaning effectively the same range although some people argue that since the Extreme can be placed anywhere and the Express must be attached directly to a power outlet, the Extreme may be set up to have a slightly bigger range. I don't really agree with this theory though.
The two have the same network features AFAIK too.
In short, the Express is much better value though.