A ranger is a unit capable of tracking people.
Rangers may also be forest-capable units, which would aid in tracking other units through forest hexes.
Captain Ford was a Ranger and a Warlord, whether Rangers are popped as warlords or are promoted to gain that special is unknown.
Rangers are capable of tracking units. When Jillian orders Jack to track a unit, he claims that he isn't a Ranger and needs to be instructed on how to do itIPTSF Text 72.
Rangers may be a type of Scout, or an entirely different class.
Rangers with Leadership may be able to "share" their forest-capable special with the units they lead and reduce or remove movement penalties for going through forest hexes.