The jungler is an interesting role in League of Legends. They do not gain experience from the creeps that travel in the lanes, or earn gold from killing those creeps. They gain gold and experience from killing neutral monsters that live and respawn in the jungle between the lanes. Each side has their own identical set of camps (neutrals). The act of jungling is mastering how quickly you kill your neutrals or the neutrals in the enemy jungle while also placing pressure on the enemy champions in lane. That pressure can be either trying to kill them, or simply make them waste their abilities.
Many different champions can jungle right from level 1. Some champions will jungle because they have difficulties in farming in a lane (usually melee champions that can be easily harassed, for example, Amumu). Some will jungle because they are great gankers (i.e. Shaco or Rammus), while others will jungle because they are particularly fast at it, resulting in a more experience/gold gained than while laning. Another difference between junglers is how safe they are while jungling: some champions will stay most of the time at low health (for example Olaf), which can expose him to ganks in the jungle to shut him down; others, like Warwick, will stay relatively safe thanks to their lifesteal, stealth, or movement abilities.
A major factor to consider when choosing a jungle champion is how quickly he can gank and how effective his ganks will be. Shaco, for example, is able to gank a lane at level 2 with just the Blessing of the Lizard Elder buff. Other junglers like Warwick or Akali are great gankers only when their ultimate ability is available. A ganker champion can play in a lane, but the enemy will be alerted when he leaves his lane to gank, and as such his ability to pull off effective ganks will be compromised. On the other hand, if the ganker is jungling, the enemies will never be sure on where he is and either play safely, reducing the amount of farm they get, or play aggressive, exposing their flanks to ganks. The only effective tools to combat a jungling ganker are strategically placed wards on the lane access points alongside wards on the jungler's blue and red buff and the summoner spell Clairvoyance.
When trying to jungle with a particular champion for the first time, it is recommended to create a custom game to test out different builds and their efficiency. If you barely stay alive at a certain mob with a particular route, you should attempt said route multiple times to make sure that you are consistent. Be sure to be able to clear the jungle without relying on lucky critical strikes or dodges. Also remember that in a normal game, you could be ganked in the jungle; hence, being able to kill a mob with very low health remaining is particularly risky. While making a mistake and dying during a bot game is inconsequential, doing so in a real match can put you very far behind in gold and level, and could even cost you and your team a victory.
Jungling Routes
Killing the Ancient Golem and Young Lizards first grants the jungler enough experience to level up and nearly endless mana/energy to keep using his abilities in the jungle without having to recall. Starting at the Golem, however, can be pretty risky, so champions that don't really need the additional mana regeneration are occasionally better off starting elsewhere. Another factor to consider is whether the champion needs significant mana regeneration during ganks. If so, then it might be wise to delay getting Crest of the Ancient Golem until right before the gank (for example, if planning to gank at level 4 getting the Golem at level 3 is a good idea since you will still have the buff for the gank.)
Here are some of the most common jungling routes:
Aggressive (fastest ganking with buffs, most common)
Wolves → Blue (Smite) → Wraiths → Wolves → Small Golems → Wraiths → Fountain → Red (Smite) → Gank
OR
Wolves → Blue (Smite) → Wraiths → Wolves → Small Golems → Red Lizard (Smite) → Gank
Sustained (more jungle experience, less ganks)
Golems (Smite) → Wraiths → Wolves → Blue (Smite) → Fountain → Small Golems → Red (Smite) → Wraiths → Wolves → Gank
The alternative path you would take to recover from an invasion at your blue
Map Awareness and Timing
In general, junglers need to have extremely strong map awareness and a strong team to support them. It is the laning champion's responsibility to inform their jungler when bushes are warded, or when champions have used their summoner spells before a gank. It is also important to attempt ganks so long as you are not over extending your attempt. A failed gank is not a complete failure if you force the enemy champion to use their escape summoner spells, leaving the enemy open to a future gank or to dying to the allied champion in the lane. While ganking, make sure you do not burst out all your crowd control at once, as this allows your enemy to escape when you have no crowd control left to deal.
Deciding to Continue Jungling or Gank Lane
Prioritizing ganking over jungling is extremely important. Watch the creep score count in the game panel to see which champion is starting to get too farmed. You must force enemy champions to be uncomfortable in their lanes. You do not want an AP carry or an AD carry to sit top lane or middle lane and free farm for the first ten minutes accumulating lots of gold. If you see a particular champion getting too much gold, you should priortize ganking that lane as much as possible.
You also want to gank a particular lane.
Timing Summoner Spells
You should know when the summoner spells are 'down' or on cooldown for the solo lane champions. If the flash spell is down, you are more likely to get off a successful gank in lane. This means that the enemy solo lane champion has to now play passive, which should mean that your allied champion can put additional pressure.
Jungle Timers
Red/Blue buff: 5 minutes
Dragon: Respawn 6 minutes
Baron: 7 minutes
Wolves, wraiths, small golems: 1 minute
Counter Jungling
When you counter jungle, this is the act of invading into the enemy jungle (crossing the river that sperates the two sides). This is very typical in mid lane, when the mid lane AP carry will go and steal the wraith camp in the enemy jungle. Counter jungling can be done by allies on your team, or by you - yourself.
Counter jungling is very effective whe you know that you will jungle much faster than the enemy jungler. For example, if you are udyr or mundo or shyvanna; and the enemy team has a jungler like skarner, xin, warwick - you will be much faster at clearing the creeps than them. This means that you can enter their jungle and steal their jungle creeps - denying them of experience and gold. They also waste time traveling to that camp and realizing they have been counter jungled - meanwhile you are onto the next creep camp or ganking a lane - this sets the enemy jungler far behind.
In addition Counter-jungling requires an increase map awareness and timing on the spawn times of buffs. When a neutral creep camp is killed, it will respawn at a given time. Memorizing these spawn times means that you can be there in advance (seconds earlier) than the enemy jungler who will only begin traveling to the camp once they see it appear. The time it takes to travel to that camp; you can have it cleared and be moving on. Timing is PARAMOUNT to any professional jungler or pro gamer.
Getting Counter Jungled
The best way to avoid counter-jungling is to have your support at level 1 place a ward at the entrance to your jungle. Based on yoru knowldege of them entering yoru jungle ; you can have your mid or even yorur bot lane come and invade your own jungle to get a 3v1 or 2v1. This will stop it dead in its tracks.
If they are counter jungling you hard; you can counter - counter jungle and go into their jungle and steal theirs if you want ot avoid a team fight (if the enemy team has a better early game team fight).
Common Jungling Items
There are not set items you should always purchase, but moreso items that are commonly used. Becuase there are so many different types of junglers (some that do damage with ability power, others attack damage), these items are not a "one size fits all". In addition, some are tanky while others are squishy. If you would like to learn more about the item builds for certain junglers, you should find a jungler you like and then read that particular champion guide here on LolPro.com.