What happens in Mass Effect 3's ending?

There are three solutions to the Reapers.
1. Destroy them, but realize that some time in the future it is likely that synthetics and organics will go to war again. (Or at least that's what the Reapers believe.) This ending destroys the mass relay network, EDI, and the Geth. This is the only ending Sheppard can survive, but only if your military is high enough.
2. Control them. This kills Sheppard, but him/her consciousness takes control of the Reapers and they fly off.
3. Synthesis. Make everyone a combination of synthetic and organic components. This negates the purpose of the Reapers and they fly off. This ending also kills Sheppard AND destroys the mass relays.

I think one of the things that really annoyed people was that the ONLY thing that modifies the endings is the effective military strength. A total paragon can chose to control the reapers a total renegade can destroy them (though the game presents the options the other way around.) Every decision you have made in this game and the ones previous have impact only through your military score.

The other thing is that the ending is very short and very vague.
1. The Crucible (the superweapon doing this) fires up, displaying the color of your choice.
2. The reapers on Earth either blow-up of fly off.
3. The Normandy-2 crashing on an alien planet.
4. Joker and your favorite crew-members get off the ship.
5. And finally one scene at the end of an old man telling a child a story about 'the Shepard.'

The ONLY thing that impacts those decisions is your level of military strength and final choice, and even then the differences are fairly minor. You have no idea what happens to your crew, trapped on alien world with a destroyed star ship, what happens to the galaxy, what happened to your companions on the final mission, or what happens to the critically wounded Sheppard if you take the only option that allows him/her to survive.

You fight 3 husks and Marauder Shields with a pistol. Then you get transported up to the Citadel. You resolve some things with Anderson and Martin Sheen. Then you get transported to another area where you get to decide with what color explosions you destroy the mass relays, stranding most everyone on Earth. Joker and your ground team that was magically put on the Normandy crashes on some alien planet where Joker has to repopulate the universe with either EDI or your love interest.

Oh, and if you imported Mass Effect 2 data, you get to see that the whole series was just an old guy telling a kid a story with sex it while they look at the stars.

You have three choices. You either control the reapers ( "blue"), Synthesis everything ("green"), or Destroy the reapers ("red"). When you build your EMS ( Effective military strength) You have an increased chance of saving everyone but in every ending the relays are destroyed. if you have less then 4000 EMS and you do not attempt to save Anderson, you won't have all the options OR the earth may be the only thing that lives and everything else is destroyed. Apparently the "red" ending or destroy option and having over 5000 EMS is the "best ending". It's really badly done, they're so many gaps and questions that you wonder if Bioware pulled it out of its ****. I was offended that they thought it was an appropriate ending/s for a wonderful game.

What happened to everyone??
What about the relays?
How did Joker get on the Normandy?
Was this a shitty dream and I was still on the bottom of the citadel the whole time?
Where's my crew and teammates?

Questions like that.

Basically you try to run to the beam to get to the citadel to open it for the crucible and then you are hit by Harbingers beam and you wake up with a pistol moving slowly and three husks come out and you kill them then marauder shields come out and you shoot him.

Then you go through the beam up to the citadel where Anderson said he followed you up and you are surrounded by dead humans then you move up into the center with shifting walls and then you go up a ramp structure where you find anderson at a control panel and he is mind f***** and is barely standing.

The Illusive man comes saying he figured how to control them and you tell him that he cant do this and he takes control of your mind and you shoot anderson and then you may persuade him to kill himself or he will put a gun to anderson's back and you get a renegade option to shoot and you can shoot or he will die.

The illusive man dies and you lay down, but the Crucible isn't firing so you get transported on a platform up to the center of the citadel after you open it. This ghost child comes and says he is a part of the citadel and he is catalyst and that you will have to choose to control, destroy, or synthesize he also said he created the reapers and then you can choose.

Destroy: all reapers destroyed and this is the only option in which shepard may live and the ghost child disappears and then a red blast is emitted

Control: you control reapers and they leave earth however shepard will die. the blast emitted is blue

Synthesize: You combine all life forms in order to make peace, shepard dies, and the blast emitted is green

in all of them the mass relays are destroyed and the normandy gets shot down by a mass relay blast and land on an earth like planet where joker and your two most frequently used squadmates come off and look on into the distance and then credits roll.

Hoped this helped and wasn't too long fell free to message me with further questions.