I feel like I need a flowchart to keep track of how many iterations Binding of Isaac has lol
There are two "branches" of Isaac
"Original Isaac"
Binding of Isaac is the original Flash game
Binding of Isaac: Wrath of the Lamb is DLC that adds more bosses, more floors, more items, trinkets, a lot of stuff we take for granted these days
Binding of Isaac: Eternal Edition, a free DLC update released after Rebirth was released. Adds a new hard mode with "Eternal" enemies, along with an addition trinket "The Game Squid." It's extremely poorly balanced and not a good addition, but can mostly be ignored.
"Rebirth Isaac"
Binding of Isaac Rebirth is a remake of the original game + Wrath of Lamb. On top of that, it adds an addition DLC size update to the original/WoL content to add way more items, characters, etc. Nothing from the Eternal Edition is included (obviously since EE came out after Rebirth)
Binding of Isaac Afterbirth is DLC for Rebirth that adds new characters, a new game mode, more items, enemies, etc, the usual stuff for a DLC update. Quite a lot of useful quality of life changes as well.
Binding of Isaac Afterbirth + is DLC for Afterbirth. So it's DLC for DLC. It adds new enemies, items, you know the drill. Also some additional quality of life tweaks. On the PC it adds official mod support. Every few months new content is added via making a few mods official.