A lot of the same, plus a bunch of new stuff. Afterbirth is a good DLC, get it if you can.
Afterbirth+ is a DLC for a DLC, only get it if you are a masochist, it ramps up the game difficulty, adds several useless/repeated items and pretty much ruins a lot of the fun stuff.
Antibirth is the best mod around, like Huff said, get that instead.
Also, TF2/Overwatch confirmed to be in the same timeline.