The ranger is one of the Advanced Dungeons and Dragons character classes that probably should not be a class in the first place. It is nearly identical to the fighter class. I am only discussing the AD&D Second Edition ranger. Your ranger from another edition or game may be entirely different.
The ranger is an outdoorsman by nature. They are usually portrayed as woodsmen, but individual rangers may have a wide range of prefered territories. Some may come from the jungle (like Tarzan), while eskimo cultures may have arctic rangers, and so on.
Rangers usually wear only light armor (with studded leather armor being the most popular choice), will undoubtedly carry a wide range of survival gear. Upon reaching a certain level, a ranger will attract a band of followers (the tables in the Player's Handbook are biased towards woodland style rangers, and should be adjusted if your ranger prefers the desert or other area), that will follow him fairly loyally (these are over and above any henchmen he may have).
Rangers are adept at hiding and skulking around in their natural habitat, (represented by a couple of thief abilities). They also have the ability to fight with two weapons much better than an equivalent fighter (which was devalued by the two-weapon fighting style made available to all warriors in The Complete Fighters Handbook). Finally all rangers have one chosen type of enemy that they gain a bonus against when attacking. The ranger is allowed to make this selection once, and may never change it. (Rangers in the first edition AD&D rules did not get to choose, instead they go their bonus against a long list of humanoids and giants).
Rangers must be of good alignment, and advance slightly slower than fighters do.