ESO like many other MMO games has many PvE areas where only team can reach success. Such areas are called Public Dungeons or instances. Players get together and try to kill bosses and loot something cool. Top weapon, amour and other items can be looted there.
Here will be a full list of instances in Elder Scrolls Online
Mass PvE battles require maximum attention and ability to play in a group. The more you use synergy the better your results will be. The game requires to interact with others to achieve maximum efficiency. Elder Scrolls Online is not about beating the area. It is about mastering your skills. When you enter the area (instance or dungeon) your enemies know about your presence and will try to kill you and your party. When you fight against monster in ordinary one on one PvE battle the monster uses all his skills against you. But a group of monsters in a dungeon will use synergy combining their skills. You will see their interaction. As soon as you walk into the room monsters begin their own strategies against your party. You will have to be quick and react properly.
When you enter the area monsters start to react. You will see it on the screen. For example someone will blow a horn and enemies will turn their heads to see you. Then they will start their mechanics to protect themselves and kill your party. Elder Scrolls Online is a game where monsters can also interact with each other. A rogue can place a trap and a mage can burn the area so when you enter it you will be caught and will receive damage. ESO requires you to be careful.
One more interesting thing: you will have to clean the whole room in a dungeon not one group of enemies. Most MMO games let you to kill 2-3 enemies in a room without attracting attention of others. ESO does not allow such trick. When you enter the room your enemies know that you are here. The whole room starts to attack your party. It is an endurance and strategic fight.
One of the big differences between ESO and other games is that there are no strict roles for players. There are no tanks, healers, damage dealers, buffers. Roles are not so strict. If you can not kill all monsters in a dungeon that means you can not interact with others properly. Use synergy. You will have fewer troubles and get more rewards.