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Rebirth is the base game, while Afterbirth and Afterbirth+ are expansions, the latter being the definitive version of the game. The title is apt not simply because it comes after Rebirth, but also because the titular Isaac is an infant.
Personally I find the game to be one of the most engrossing I've played - I've logged over 400 hours into various releases dating back to the original Binding of Isaac, and I've still only unlocked about 60% of Rebirth/Afterbirth+'s achievements. However, the title is probably one of the least offensive things about the game - matricide, suicide, and religious torment are all big themes, all wrapped up into a pretty little 2D Zelda/bullet-hell/roguelike mashup.
XBL: RandomTag529 | PSN/Steam ID: FEGuy | NNID: JimBobIX Core i5-6500 | GTX 1070 OC | 16GB DDR4-2400 | Windows 10 Pro + Arch Linux