the engineer can build stuff:
1. sentry guns (3 levels)
2. dispensers
3. teleporters (entrance and exit)
you can upgrade/repair your sentry gun by whacking it with your wrench (as long as you have metal available, metal can be acquired from ammo boxes, picked-up weapons, dispensers and of course from the resupply closet).
other damaged things can also be repaired by whacking them

a dispenser generates metal and can be used to heal yourself (or others)
a teleporter is just a teleporter (note: you cannot use a teleporter when you're carrying the intelligence)
the main problem for engineers are spies. they can "sap" your devices, which makes them explode after a few seconds. you can remove a sapper by whacking your device twice (of course this leaves you open to the spy that wants to backstab you while doing that).
TF2 is definately the best game I've played in a long time and has the most humour
