Difference between a code and geass? Code seems to be obtained after your geass upgrades to full power and you kill a code bearer. What happens to your geass after you obtain the code? Do you lose your geass? Is code an extra power? What is the difference between these two?
When the Code is obtained, the one who obtains it seems to lose their Geass Powers. We see this happen with C.C in her flashbacks as beforehand she had the Geass Power to make everyone love her. In the scene where we last see with her Geass her Geass is now uncontrollable before the Nun admits she tricked C.C, from then on we see C.C's Geass Sigals on her forehead instead of in her eyes.

We can assume the same thing happened with Charles. At the end of the first season he uses his Geass to manipulate Lelouch's memories but he didn't have V.V's Code yet as later on in the second season when Cornelia infiltrate's the Order's Base she throws a knife at V.V seemingly killing him until he gets up and takes the blade out of his head. However at the Sword of Akasha after Charles "comes back to life" he doesn't once uses his Geass on Lelouch.

We also know that the Code doesn't have to be given/received willingly. The Nun who gave C.C the Geass powers forced her to take the code (though we are unsure how) and in the second season as C.C is about to reach the Twilight Gate V.V is seen wounded saying Charles "stole" his code

In terms of what the Code Bearer gains, from observations we see:


Ability to create contracts with others to bestow Geass
Immunity to Geass
Immortality


There is another power we only see once in the first season and used by C.C at the Battle of Narita where C.C feeds Suzaku "shock images" to immobilize him. However I am unsure if this is a power related to the Code or a natural power C.C has as neither V.V nor Charles demonstrates such a power.

Also, the Twilight Gates can be accessed by anyone who has Geass or the Code as Charles is seen at the Sword of Akasha without V.V with him at the beginning of the second season (where he first shows Suzaku the Sword of Akasha saying it's a weapon to slay God)