League system

League system is a ranking system to find other players of a similar skill level to play with or against. In seasons one and two, the system used a single ladder with ranking determined by Elo rating system. In season three, the system uses six tiers with five divisions per tier, where ladder rankings within a league are determined by League Points (LP) and hidden Match Making Rating (MMR).

League Points
Each ranked game you win earns you League Points while each lost one decreases your current LP. The exact amount of lost or earned League Points per game depends on your hidden Match Making Rating. If you have higher MMR than a set amount for your division you gain more LP per won game and lose less LP per lost game, if your MMR is lower you gain less and lose more LP per game.

There is also League Points inactivity decay over time, by a set amount, if you haven't played ranked in 28 days. It decays per the set amount every 7 days afterwards until you play a ranked game. The amount of LP that you lose per decay period depends on the tier that you're in: Notes:
 * Bronze - 0
 * Silver - 10
 * Gold - 25
 * Platinum - 35
 * Diamond - 50
 * While you're inactive, you're hidden from the ladder view for everyone except yourself.
 * Match Making Rating doesn't decay when LP decay occurs.

Tiers and Divisions
Each tier (except Challenger) contains five divisions, each division is numbered using  in a descending order from V(5) to I(1), with V being the worst and I - the best.

Tiers from the lowest to highest:
 * Bronze
 * Silver
 * Gold
 * Platinum
 * Diamond
 * Challenger Tier

Divisions from the lowest to highest:
 * V
 * IV
 * III
 * II
 * I

Promotion
When you reach 100LP you start your Promotion Series, where you have to win 2 out of your next 3 ranked games. If you win your Promotion Series you are promoted to the higher division and your LP is set to 0, if you lose your Promotion Series you remain in your current division with 70-90 LP (based on the amount of wins in your promotion series).

If you reach 100LP in division I, you begin your Tier Promotion Series, where you have to win 3 out of your next 5 ranked games.

If you leave during champion selection or in-game it counts as a loss on your current Promotion Series.

In very rare occasions, it's possible to be promoted twice after one Series, due to having a high MMR. For instance, someone in Gold V that duo queues often with his friend in Platinum III might have an unusually high MMR due to the players that he is matched against. As a result, his MMR may be 1 or 2 divisions higher than the average MMR of his current division. After winning his Promotion Series, there is a possibility that he may be promoted twice. Dodging queues may also, to a smaller degree, affect the difference between your MMR and LP (-3LP per dodge).

I am a nub

Demotion
Demotion happens when your current League Points reach 0 and you lose enough games to get demoted. When you are demoted you go to one division below your current and get 75 LP in that division e.g. if you are in Gold II and get demoted you will fall to Gold III and have 75LP.

There is also a demotion immunity grace period for several games, in which you can't get demoted to a lower division, which activates when you get promoted to a higher division. This is to make sure you don't fall to your previous division due to bad luck.

You can't be demoted below division V of your tier unless you fallout to a lower tier due to inactivity LP decay.

During server issues losing a game and getting "Loss forgiven" prevents demotion and will not count as a game for Promotion Series - you won't lose them due to that game.

Challenger Tier
The Challenger tier is the highest tier in the League ranked system and consist of the most skilled and competitive LoL players. Due to the competitive nature of this tier there is no LP inactivity decay but each time you gain the LP to move up one spot in the Challenger tier ladder someone gets pushed one spot down.

If a player reaches Challenger tier in solo/duo queue even if they don't have a ranked team they are likely to be spotted and possibly recruited by a professional team.