Bossing in Path of Terraria is different from how the Vanilla game handles bossing. All bossing is done in the “Bosses Domain”.
Boss domains are unlocked through Questing and their respective vanilla methods, such as summon items, are disabled or unobtainable. This includes alternative summoning methods, such as Abeemination, alongside the main summon method, such as breaking Shadow Orbs or challenging the Old Man's curse.
When fighting a boss domain, usually, you are given 6 lives. Lives are determined by the players that joined the boss domain through the portal. All player always get at least 1 life though.
Singleplayer:
You have 6 lives.
Multiplayer:
2 players: Enter world, you both have 3 lives.
6 players: Enter world, you all have 1 life.
12 players: Enter world, you all have 1 life.
There are a few exceptions to this. For example, the Wall of Flesh domain has a strict 1 life per person, regardless of how many players you have.
If a player dies and has no lives remaining in the domain, they are turned into a ghost until the boss is dead or everyone else is. At this point, a short timer begins and players return to the main world in unison.
Bosses are split by progression groups (separated by horizontal lines). Everything in a group can be completed in any order; but must be preceded by the stated amount from the prior group.
To begin the quest system, you must start with the following boss:
Any three of these can be completed in order to progress.
Eater of Worlds or Brain of Cthulhu
Queen Bee
Skeletron
Deerclops
Wall of Flesh