MOBA is short for Multi-Player Online Battle Arena. As a general rule it is a combonation of an RTS or Real Time Strategy, and a MMORPG or Massively Multiplayer Online Role Playing Game. They are generally centered around the idea of player versus player activity, and in many typical MOBAs, a player is responsible for only one character at a time, often referred to as a Champion.
League Of Legends as A MOBA.
League of Legends falls under the catgeory of MOBA because it fits the exact description of one. In League of Legends, you control one champion, on a team with other players also controlling one champion. These champions face off against eachother to accomplish a goal that will ultimately grant them victory. It is played from a top down perspective, and involves mechanics such as Fog of War.
In League of Legends specifically, you control a champion who purchases items using gold collected by either killing champions, killing minions, destroying towers, or completeing other objectives. Players also passively gain gold over time. All of this will be covered in more detail later on in the course of League of Legends Noob to Pro. These items augment a champion's natural stats and help them in objectives such as Destroying Towers, Taking Buffs found around the map, Defending Towers, Destroying or Defending Inhibitors, and Destroying and Defending the two Nexuses in the game. Some of these actions are only available in certain game modes, and some game modes offer additional objectives and actions, however the majority of the game is played with these things in mind.
What makes League of Legends Different from other MOBA games?
League of Legends offers an ever expanding game. Because of its free to play business model, new content is constantly produced in order to generate more revenue for the game's creators. This means that players must constantly adapt to new champions becoming available in the game, which brings with it new game mechanics, and new ways to win. Furthermore League of Legends regularly recieves patches, or changes to the game's mechanics and statistics for items and champions, in order to achive and maintain balance, and fair gameplay.
League of Legends' free to play model also means that the game is available to everyone, and boasts a very high number of players. Meaning that no matter your skill level, or time zone, you're able to find a game. The current estimate for recently active accounts is around 32 million, or .5% of the world's estimated population. This is roughly one in every 200 people. However many people run multiple accounts so these numbers are slightly inaccurate.