WARNING: CONTAINS SPOILERS
Although this is brief, this is all its going to take to give you the ending (and i'm not kidding the bit in bold at the bottom is the ending movie)
So now that the game has been out long enough and everyone is complaining about the ending. There is a good reason for this as even absolutely CRAP games have better endings than this.
So you go through all the trouble of trying to find the said 'catalyst', the catalyst of course is the only one place you ever go to more than once, the Citadel . Your final fight takes place on earth, where you need to make it to a beam which connects earth and the Citadel, this is the only way onto the vessel as it is under reaper control and closed. So you make it to the beam, although everyone around you is dead.
Get beamed up, you walk a little, you see Anderson at a control console, then the famous illusive man shows up, he says he can control the reapers, but you say he can't etc. He shoots Anderson, you shoot him. The protheon device is then joined to the Citadel.
You then meet the young boy who was killed on earth, he is the catalyst, you are then given the choice to either control or destroy the reapers.
I chose to destroy them, so the ENDING!!!
You see a ball of fire get shot out of the Citadel, this destroys all life around it, then a beam shoots out of it destroying all the mass relays. This beam is catching up to Normandy, the next scene is the Normandy crash landed and the door begins to open...CREDITS...After the credits you see an old man telling a story to a child and THAT'S IT. I will upload this movie so you can have a look