Summoners

The Summoner is the persistent element in the game, and it is used to track statistics and scores for each player.

A player in League of Legends takes on the role of a summoner, a gifted spell caster who has the power to bring forth a champion to fight as their avatar in Valoran's Fields of Justice. With all major political decisions on Valoran now decided by the outcome of the contests that take place in the battle arenas, a summoner is the key force of change on the continent. As the summoner controls the champion in combat, the summoner also influences the outcome of the match through the use of summoner spells, runes, and masteries. Once a match is over, the summoner gains experience and influence points. Experience points allow the summoner to gain increased powers by unlocking additional spells and rune slots, while influence points allow the summoner to gain access to new champions and new runes for use in future fights.


Customization

The summoner will represent a player and track its stats as games are played. As you progress in the game, the summoner will accumulate Influence Points allowing access to more champions and runes.

Summoner spells

Summoners are allowed the use of any two of the summoner spells during any battle in the Fields of Justice, many are available from the start while others are unlocked through gaining levels. These spells significantly impact gameplay, and have a high cooldown while costing no mana. All spells can be improved by masteries.

Runes

Before a battle arena match begins, a summoner can make use of Runes to enhance their champion's abilities. Runes can only be purchased with Influence Points at the Riot Store. Runes may not be bought with Riot Points, this is to help prevent people from buying power.

Masteries

Masteries are perks that affect gameplay, they are commonly referred to in other games as "skill-trees". All of the masteries are passive effects although some augment summoner spells, which can be activated. A summoner is able to focus their intense magical training into specific categories (Offensive, Defensive, and Utility). Each group has 6 tiers, within a group all but the first tier are locked when you first start, with a successive tier opening with each 4 points spent in the same group. Lines in the mastery tree imply additional unlocking criteria. The summoner can put one point per level (up to level 30) into masteries. Masteries can be re-distributed at will between battles. Since May 23, 2011, summoners are allowed to have multiple saved mastery pages.

(Source)