There are three roles for the jungler. These roles are EXCLUSIVE, you can only do one of these and there are junglers that are good at each.
Role 1 Ganking - The high risk, high reward early game strategy. You farm to level 3 as quickly as possible, gank lanes over and over, trying to get lanes ahead. You want champions with high base damage, high mobility, and quick clear speed. Elise, Lee Sin, and Graves are examples of this.
Role 2 Control - A medium risk, medium reward role that aims to retake the game in the team fight stage. Rather than ganking, you try and invade each time the jungler reveals himself. You take his camps and set him back in XP and gold. The jungler focuses on providing team buffs to the group with dragons, Gerald and Baron. You Sneak these, and so you must be able to solo epic monsters very early. Nunu, Skarner, and Trundle are examples of this.
Role 3 - Farming - The low risk, low reward late game role. You clear camp's constantly, base and buy more stars to clear faster. Farm and farm and farm. You get up to three or four items with no one to stop you, and then you come out of the jungle to 1v5 carry. Master Yi, Jax and Shyvana are examples of this.
Now in previous seasons, ganking countered farming (you lost the game before you hit your power spike), control countered ganking (you got your laners ahead but you were totally useless), and farming countered control (usually control junglers are tanks who get slaughtered by duelists, the farmer clears his jungle faster than the control can invade). Now, however, the creation of Stalker's Blade (Smite as a combat spell that steals MS) and the removal of Sight wards (ganking results in kills more frequently), camps respawn more slowly (you can clear all of them and then they dont respawn for you to keep farming) and dragons no longer give team gold, all of these things result in ganking being the strictly best strategy.
That said, fuck your team mates. Play whatever game you want. If you want to gank, do it when you want. If you want to farm, tell your team they get no ganks.