Dungeons may be entered by approaching the portal and selecting a mode and difficulty. The four modes that are available for each dungeon are Story, Elite, Heroic, and Legendary. Story is a mode with no difficulty settings, and is required in order to progress the main questline.
Story mode is the easiest mode, and is meant for progressing the player through their Story-Quests. Each Dungeon has its own player limit. Once a dungeon has been completed in Story mode, the next difficulty up will be made available.
Elite, Heroic and Legendary modes have variable difficulty levels between I (Very Easy) and V (Very Hard), with higher drop rates in more difficult modes. Elite is the first challenging mode. Heroic is the second difficult mode. Legendary difficulty is the most challenging, and is recommended for well geared players.
Raid Dungeons also exist that will only have one difficulty setting. These dungeons are aimed at geared and coordinated team members to overcome challenging foes.