There are 2 types of Rangers who are differentiated based upon the area in which they work. The Rangers of the North, and the Rangers of Ithilien or the South. Both groups were founded a few centuries into the Third Age of the sun by Turin II the ruling steward of Gondor, who deemed that Gondor Need protection from the threats of Mordor and Minas Morgul in the North. And also the various factions of Orcs and Easterlings in the South. He formed a brotherhood of knights who would fulfil these tasks.
Rangers of Ithilien
During the War of the Ring both rangers were called into battle for different reasons at different times.
Faramir and his rangers were summoned by Denethor to hold off the orcish invasion of the former Gondorian capital of Osgiliath. This however was a very poor decision as the rangers had very little experience at fighting with others in the front line, and even less at fighting in urban enivornments. As a result they suffered terrible losses, losing nearly half their number. This, however was not sufficient for the insanity of Denethor who ordered the remaining 100 rangers to ride out with infantrymen to attempt to take control of Osgiliath, which was now hosting close to 100,000 orcs, and several nazgul. They never got close. Gothmog, leader of the orcish forces, ordered 5000 orcish archers to fire on the riders. All the riders died save Faramir who himself suffered extensive wounds from arrows. This led to the decimation and collapse of the Rangers of the South.
Prior to this, Faramir found Frodo, Sam and Gollum, who all had witnessed one of their ferocious attacks on the Easterlings. He took the Hobbits to Henneth Annun where he discovered Frodo had the One Ring. However showing resilience to its power far greater than his brother he let the Hobbits go on and complete their quest.
The Rangers of the North were largely more succesful because they had been called in by the King during the battle of Pelennor fields. Aragorn who had an extensive knowledge of the rangers and how they worked used them far more strategically than Denethor. As a result the Rangers suffered comparably fewer losses than their southern counterparts.
These knights were garbed in green, and fought with bows and spears and swords. There primary area of concern was the road linking Minas Morgul to Osgiliath. They specialised in warfare not disimilar to current Guerilla tactics of stealth, sabotage and subterfuge. They planned ambushes to take out battalions often several times the number of the few hundred Rangers. They were the most feared of Gondor's warriors. Their stealth and subtelty is a testament to their training programme because unlike their northern counterparts, to them elves were simply a thing of stories, whereas the northern rangers had 2 elves as members as well as their captain-Aragorn-having had extensive training under elves. So the fact that their stealth is on par is a tremendous feat. Their base of operations was a dwelling of caves and tunnels that lay behind a waterfall which overlooked the Vales of Anduin. This was called the window of the sunset or Henneth Annun in elvish. During the War of the Ring the captain of the Rangers was Faramir son of Denethor the Steward of Gondor and brother of Boromir who was a member of the fellowship and a captain of Gondor. Faramir has been called the greatest of the Rangers which is an impressive feat especially considering his competition was none other than Aragorn, a member of the fellowship who would later become King of Gondor.
Rangers of the North
These Rangers operated very differently from their southern counterparts as there were far fewer threats to the North. As a result of this they needn't travel in the small packs and employ stealth and ambushes as frequently. Instead they merely needed to make their presence known to all who passed in and around Gondor for intimidational purposes. In this respect they were tremendously successful. They are described as beig clothed in cloaks of forest green or grey, and on their left shoulder each ranger carried an emblazoned star. They were grey eyed and wore long leather boots. To all the people's of Eriador (where they patrolled) they were regarded as strange, unfriendly people. Their intimidating presence was heightened by the mystery that surrounded them. No one knew where they had come from or who they were, merely that they rode silently on great horses. Few realised that these rangers comprised of Nobles. These include famous and powerful people such as Aragorn, heir to the throne of Gondor. The two sons of Elrond: Elladen and Elrohir. As a result of this elvish insertion into their ranks their skill and proficiency in skill, stealth and archery grew rapidly.
Major Story Arcs
Post War of Ring and Fourth Age
For the Rangers of the North this was their golden age for they were rewarded generously by the King, and so lived a comfortable life. The Rangers still continued, and Aragorn resumed his leadership, however on a much smaller scaled due to the fact that the number of threats were smaller and the magnitude of them insignificant.
At this point the Rangers of Ithilien were practically non-existent due to the massacres they'd faced during the War of the Ring under Denethor. As a result Aragorn set about rebuilding them. Faramir was re-instated as Captain as well as Prince of Ithilien. Their prowess did not decrease however the number of threats they faced did.