So we have four potential endings we could have had;
1) Super duper everything is hunky doory ending. Final battle on Earth has Quarian armadas laying down fire from the skies to clear the frontlines, dropping in allied Geth units to clean up stragglers, meanwhile Turian warships bring in armies of Krogan soldiers to watch your back. Volus, Hanar and Elcor warships show up to do what they can. Your allies and friends from the last two games lead assaults on various fronts. It's the culmination of all your effort to unite the species of the galaxy and forge alliances, in unison fighting until the bitter end to take down the Reapers. Shepard and Anderson confront TIM on the Citadel and in struggling from wounds caused by Harbinger activate the Crucible, sending out the Reaper killing shock wave across Earth, using the Relays (intact) as peer-to-peer signal amplifiers to purge the galaxy. The Citadel plummets to Earth. Nobody suspects you to have survived yet, from smouldering ruins they find you. Alive, barely, but alive. Your faithful buddies carry your broken body from the ashes, fade to black. Time to heal, rejuvenated, and the galaxy celebrates their moment of absolute triumph. Brief cuts to each homeworld show the state of things as you left them; the Asari grieving their lost homeworld, the Turians rebuilding, the Geth and/or Quarians revitalising Rannoch, and the Krogan empire triumphing or perishing. You alive, your friends alive, the future restored: but still plenty of mystery as to what will happen next.
2) Middle ground. Shaky alliances and a few poor choices cause heavy causalities. Some good friends die, but there's not much you can do. Warships drop from the sky. Species are bought to the brink of annihilation. Not everybody will recover. Yet, you prevail, but at a cost. Shepard's activation of the Crucible is something he/she cannot survive, and his/her body is lost as the Citadel plummets through the atmosphere. Earth is devastated, The universe is building. But people will rebuild. Wounds will heal, in time. The species gather to salute and say fair well to the fallen hero who was the key to ending the Reaper threat, then return to the homes to clean up the rubble, bury their dead, and begin their new lives.
3) Failure. Failed alliances. Enemies at every turn. Dead friends because you're a stupid dumb idiot. Nobody works together, the armada isn't strong enough, and the ground forces are completely decimated. Shepard fails to activate the Crucible as the Reapers break through Alliance warships, ripping it to shreds. It is lost. Everybody tries to flee. Most are caught and destroyed. Some escape, though are doomed to be hunted down, as the galaxy is drenched in an organic harvest. Worlds burn, and it ends. Credits roll. Post-credits sequence of an unidentifiable ship landing on a strange world. A mysterious alien species, unlike anything you've seen before, exists the ship. Digging through the dirt, they unearth a strange black box. Suddenly it lights up, a holographic image of Shepard appears, and Liara's voice can be heard, issuing a warning of things to come. End.
4) EDI, Tali, Liara and all the sexy alien babes have sexy time and it's really cool.