Basically, forwarder is a specially equipped tractor which is used to haul logs from woodland to depot.

These tractors have a compact trailer and crane unit for loading the logs onboard.
Usually forwarders don't have front wheels in the same sense as farm tractors do. Instead, pivot steering is used (the trailer is coupled to the tractor by means of a "hinge" and strong hydraulic cylinders steer the tractor).
Modern forwarders have automatic transmission, air-conditioning and power-steering. They are powered by strong turbo-charged diesel engines and they can carry very heavy loads of timber in most difficult terrains, too.
http://www.taimetsoi.fi/uutiset/what_is_a_forwarder.htm