Defense of the Ancients, or (DotA) for short.
The first well known MOBA game ever created, originally a custom online game mode in Warcraft III. It had a continuation (DotA 2) that is one of the top MOBA games in the world. Many things in 300 Heroes is similar to Defense of the ancients, including the style, map, some characters, powers, items, game functions, terrain, game flow, etc.
Some champion pages or item pages will assume you have a cursory knowledge of champions from Defense of the Ancients in order to make comparisons simpler between different 300 heroes playable characters and Defense of the Ancients playable characters.
An incomplete list of characters that are similar or nearly identical to Defense of the Ancients characters can be found Here.