Hi guys,
I started a few days ago my journeys on GD, and I would like to know something about EndGame. I just finish the campaign in normal difficulty, shall I finish the campaign to Elite or can I move to crucible or something else? The question is, there is another way to reach the lvl 100, or the campaign is the only way?
Welcome to the forum and to the game. 
You can switch between Crucible and the campaign whenever you feel like if you want to. The Crucible is a separate add on battle arena mode where you fight waves of foes. When you cash out you get your loot.
Campaign is pretty much it unless you love Crucible so much you level all the way up in there. If you have the Forgotten Gods expansion then you could also play around in the Shattered Realm endless dungeon if you want to.
Thank you so much for your reply! There are some quests that I must do in order to unlock all features in the EndGame?
So Endgame is essentially:
Nemesis - kill badguy factions until you get to Nemesis level which spawns special monsters…
Rogue Dungeons - Skeleton Key to farm for more gear
SR/Crucible - for gear and bragging rights
Celestials - Probably don’t even want to ask until you get deep into #3.
All of these are ‘behind’ an unlock (quests) but they’re all fairly separate from the campaign except for #3 which requires you to reach a certain point in the campaign before you can get to it.
This is all for Ultimate difficulty.
EDIT: Forgot to state the obvious endgame activity…leveling alts. 