The term "ranger" may refer to a type of character from one of several canons.
An elite fighting force mainly comprised of Minbari and Humans specially trained on Minbar during the Shadow War. Nurse Mirrad is one.
Dungeons & Dragons
"Ranger" is a character class in the game Dungeons & Dragons. It draws heavily on the Lord of the Rings idea of a soldier-woodsman, but also has connotations of being even more connected to nature than normal people (but not as much as Druids). They are often followed around by animal companions, which are sometimes Cute Animal Friends. Drizzt do'Urden was a Ranger.
"Ranger" is a character class in a few Final Fantasy games. Essentially a bowman, they are able to strike at full force from the back row of battle in the games they appear in, whereas most would get a damage penalty in exchange for the defense advantage. In Final Fantasy Tactics, they are ranged fighters and can strike from several squares away.
Power Rangers/Super Sentai
Power Rangers and Super Sentai are 2 sides of the same coin, and both are generally referred to as "Rangers." There are several different colors of Rangers every TV season, and they all have their unique powers and giant robots to pilot. Red is nearly always the leader.
In the Ranger's Apprentice series, the Rangers are an elite group of fifty men highly trained in stealth, camouflage, knife-throwing, dual-bladed fighting, and near-unbeatable marksmanship. They are tasked with protecting their home country of Araluen.