~In no order that im aware of and this is taken from another source and not my words~

#1) The ending in theaters is as follows:

SPOILERS AHEAD

Katie, possessed, wakes up in the night and stands over their bed for a while. The sheet covering Micah is pulled off by invisible hands, and Katie goes and stands over him for a time before she slowly walks downstairs. After a short period of silence, she begins shrieking at the top of her lungs, most likely it is the demon using her body to lure Micah downstairs as Katie once implied it could set up traps. Micah rushes down the stairs, we hear him shout a bit, then silence. Next, we hear heavy footsteps coming up the stairs and all of a sudden Micah's body flies across the room at the camera. We see Katie standing in the doorway, covered in blood, moving toward Micah's body. She crawls to him and either sniffs or licks his body before approaching the camera with a super-creepy possessed face. She then lunges at it and the screen goes black. The end.

There are also 2 other endings, all with the same lead-up EXCEPT nothing happens to Micah's sheet, Katie simply stands over her side of the bed as she had done earlier in the film. The change in the theatrical ending was probably put there to express how much worse the possession had gotten.


#2) In the ORIGINAL ENDING, as in, the one that was shown in most of the earlier screenings and can be seen in most versions found online,

SPOILERS AHEAD

We hear the footsteps coming up the stairs after the whole screaming incident with Micah, except this time Katie is alone, bloody, and holding a knife. She then sits on the floor next to the bed rocking back and forth for about 2 days straight. During that time, their friend from earlier calls to ask if they're okay as she hasn't heard from them in days. Later, she actually comes to the house in the night and discovers Micah's body. Finally, cops arrive, and as they enter the bedroom to see Katie on the floor, she emerges from the possessed trance, begins freaking out, confused about where Micah is, panicking, etc...She is making very little sense and the cops are clearly freaked especially when she starts coming at them with the knife in her hand. Taking this as a threat, they gun her down in the hallway.

EDIT: After reviewing this ending thanks to a replying poster to this thread, the gunshots fired by the police that murder Katie were more a result of them being startled by a door slamming behind them. Earlier in this scene, just before Katie is discovered, we see the light at the end of the hall turn on and then off, presumably indicating the demon escaping from Katie's body.

#3) Finally, there is the ALTERNATE ENDING which was only shown at one screening.

SPOILERS AHEAD

Again, we hear the heavy footsteps and like the one with the cops, it is Katie alone, bloody with a knife, but no body. She stands in front of the camera without looking directly into it, implying she is still entranced, and proceeds to slit her own throat. This is supposedly meant to bookend Micah's comment earlier in the movie when he is cooking something, holding a knife, and jokingly gestures that they will slit the throat of whatever is tormenting them. The reason this ending was fairly easily dismissed was that the director felt the gore of it didn't coincide with the rest of the movie, as up until that point there was no gore whatsoever. However, both actors have attested that this ending was most effective in terms of audience reaction.