Up to 10 computers supported wirelessly.
1 Ethernet port.
USB port for printer sharing only.
Audio out for AirTunes streaming.
Extreme:
Up to 50 computers supported wirelessly.
3 Gigabit ethernet ports supporting a large number of cabled machines.
USB port for HDD and printer sharing (simultaneously if attached to a hub).
Express is more of a travel base station whereas the extreme is more of a enterprise or home solution.
Please be mindful the Express is the only one that supports music and the Extreme is the only one that supports a HDD attached (well except the Time Capsule, which is an Extreme with a built-in HDD).