When you interact with the door, a UI will pop up and have you select which difficulty you can play, select a key to use then you can add a raid mod and open the portal.  
As you enter the door, you'll be teleported to the waiting area, giving other people a chance to join your raid, which will also increase your chances for loot by a set percentage.  
Loot that has a beam of light above it is owned by you, if it doesn't have a beam of light, then it isn't yours.  
There are several different resources you can collect in-raid. You can collect Copper, Iron, Gold, Diamond, and Berries. Here's an example of an ore node, and a berry bush.  
Killing the Boss(es) on Wave 5 drops you Stat Mutators. Killing the Boss(es) on Wave 10 drops you a key shard respective to the tier of raid.  
Both Bosses will drop you a potion and possibly more when you kill them!

You can take 10 medical items and 10 potions with you in a raid.  
The type of loot which you earn is based on the difficulty of the raid- see the table below regarding difficulties you need to play to have a chance of getting higher grade weapons.

NormalTier 1
AdvancedTier 2
EliteTier 3
ExpertTier 4

Related Topics: 3.1 Raid Keys, 3.2 Raid Mods, 3.3 World Bosses, 3.4 Enemy Types, 3.5 Citadel