Once you played through all the 10 acts that is Path of Exile’s campaign you will reach the end game. In the end game you will find many bosses. Every map has an end boss, and you will also face bosses that can be unlocked by fragments that are much harder to fight.
To unlock these bosses, for example, Shaper, you would need to kill their Guardians. They are Guardian of the Phoenix, Guardian of the Hydra, Guardian of the Minotaur and Guardian of the Chimera. Every one of these mini bosses will drop a fragment that can be used in the map device to open a portal to the Shapers Realm.
Beside Shaper there is Elder and Uber Elder, Atziri and Uber Atziri, Sirus and the strongest boss The Maven. Recently in the latest expansion Atlas of the worlds, they have also added The Searing Exarch and The Eater of Worlds and presented us with Uber Uber versions of some bosses that only builds can do when they have put a lot of investment in them. The builds that are tagged with the Boss Killer tag can kill all these bosses except the Uber Uber versions of them as that requires a very expensive and well thought out build