There are three types of jungling strategies in existence.
Ganking, Control, and Farming.
These generally overlap somewhat. A control jungler may gank here and there, a farming jungler may try to establish control, and a ganking jungler will usually take advantage of a lul to get in some power farm.
Here is the main point of each strategy.
Ganking involves establishing an early level lead through fast clear speeds, healthy clear speeds, and mobility that allows you to interrupt enemy champions and give gold to your laners through kills and assists. This is an early game strategy that falls off hard if the game goes longer than thirty minutes, as by that point gold leads start to balance out naturally. This is a high risk, high reward strategy as it creates a level disparity between the dead champion and the free-farming champion, which will make the trades in lane more in favor of the ganker's laner, which can roll into another kill. That is what people mean when they say 'snowball'.
Since the introduction of the
, which grants the jungler a point and click true damage nuke that slows the enemy and speeds up the jungler, ganking has become less high risk while retaining its high reward.
When you combine this with the ability of an early game jungler to end the game before he falls off, and the visibility of ganking impacts (in other words, your teammates appreciate it more because it affects them directly) people tend to think of ganking as the primary or even only strategy. If you want to jungle, you need to tune out your teammates. They don't know when it's right to gank a lane, you do. If your laner asks for a gank, tell him either you're on your way after red / ping on my way or politely tell him that you won't be ganking his lane. This is especially important if the lane has already lost. If the mid laner is 0/3 and the enemy mid is 3/0, you could potentially give up tower, dragon and buffs if you gank the lane and give a double. Plus, it's likely that there's just an honest difference in skill between them. Giving a worse player extra gold won't help. He's got less skill, he won't win the fight anyway. Your time is better spent elsewhere.
Ganking jungling counters farming jungling, because if it is done successfully the hyper scaling farm jungler will be fighting against a more powerful team while his own team has lowered morale, thus making it harder for him to win the game.
Control jungling is a strategy that involves putting your team as a whole ahead while putting the enemy jungler behind (as opposed to ganking, which puts the enemy team behind and you as the jungler ahead). You accomplish this in several ways. Warding and taking camps from the enemy jungle, taking scuttle and dragon/herald/baron as often as you can, and acting as a shot caller. A large part of this job is issuing pings because your teammates will tunnel and not even use your wards in the enemy jungle, especially in the laning phase. Because control jungling involves moving into the enemy jungle, it is medium risk, medium reward.
Control jungling has become a more powerful strategy with the introduction of the new elemental drake system. Each individual dragon provides a permanent slotless benefit to your team which can more than make up for the power of raw gold and XP granted by a ganker, especially if you can stall the game out long enough to farm up equal gold. (Every dragon except Mountain will help you stall the game out).
Control jungling has had several nerfs in the past, both directly with the removal of Poacher's Knife and indirectly with changes to the jungle experience system which was intended to nerf farming junglers. It is largely invisible power, and will frustrate your teammates because they are meta sheep who think your role is all about them.
Control jungling counters ganking jungling, because if your teammates are not terrible ganking will not always result in a kill, and then you punish the enemy jungler by stealing his camps and potentially setting up a gank inside his own jungle with vision. If done correctly, you can end up as many as 2-3 levels ahead of the enemy jungler and easily 3-5k gold ahead.
Farming jungling is a strategy that involves staying inside your own jungle to try and take advantage of a largely PVE experience. You can farm up gold and experience relatively quickly without any player interaction, and then come out in a team fight with more items than the other players. Champions who generally prefer to farm jungle have hyper-scaling elements to their kits and can do a lot if they are given a harass-free gold source. This is a low risk, low reward strategy because it offers the bare minimum of interaction and the reward is basically just raw gold without the risk of being killed or ganked.
In the preseason 7, Xypherous stated in a PBE thread that Riot is actively suppressing the farming jungler strategy because they don't like it. They want players to interact at all points in the game and instead of simply cutting the laning phase out entirely and making ARAM the main supported game type, they just want to force junglers out of the jungle. They did this by drastically increasing the amount of time camps take to spawn, which means that you have a lot of down time as a farming jungler. Whole lot of doing nothing sadly.
Farming jungling counters control jungling because farming junglers will generally clear their own camps faster than a control jungler can invade to steal them, and then the game becomes about who can scale more off their own jungle (and the answer to that is clear).
Hope this helps.