There are 4 endings (1 "good" ending, 1 selfless choice ending, 1 selfish choice ending, and 1 "bad" ending). Which one you receive depends upon whether you saved or harvested Little Sisters and whether you spared or killed NPCs.
You receive the "worst" ending if you harvested every Little Sister you encountered. In this ending Eleanor brutally harvests Delta's ADAM against his will, preserving his experience and survival instincts for herself. She then watches as the bodies of dead Splicers rise to the surface of the ocean. The corpse-filled ocean, which is crashing violently as a powerful thunderstorm rages overhead, foreshadows the bloody future the world will face, as it will now confront the full might of Rapture's godlike genetic technologies wielded by the ruthless genius and unimaginable cruelty of Eleanor.