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
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)
A geass is a supernatural power, like Lelouch's ability to command people to do things, or Charles's ability to rewrite people's memories. A code is a, uh, different supernatural doohickey that 1.) makes you immortal; and 2.) allows you to grant geasses to other people.
What happens to your geass after you obtain the code? Do you lose your geass?
Yes, you lose your geass.
Recall that C.C.'s geass was the power to make people love her. Once the nun passed her code on to C.C., she lost her geass - if she still had the power to make people love her, surely she would not have been tormented and treated as a witch by the people of her time. Similarly, V.V., another code-bearer, shows no signs of having a geass.
A code and a geass are two different things.
A code grants immortality and eternal youth and allows the person to give geass powers to others.
A geass is a power which differs from person to person. It's said that it is the expression of the utmost inner desire of the person with the geass. Examples are mind control, mind reading, altering memories, forcing people to love you, etc. when a geass user uses his geass a lot it grows "stronger", which means that after a while he can no longer turn it off and it will be active at all times. if it gets even stronger it will affect the second eye of the user and from then on he can take the code from a code bearer, rendering the former code bearer mortal.
There's a lot of disinformation about this online, but the anime is actually very clear on this:
When you get a code, you lose your geass. This was shown to be the case for C.C., Charles, V.V. and the nun from C.C.'s backstory (those last 2 never displayed any geass, and since they have the code it means their geass must have been constantly active in both eyes, and yet we don't see their geass, therefore it's gone). There are no exceptions for this in the lore. And Charles even explicitly says "I've gained a new power IN PLACE OF geass".
Transferring the code does NOT kill the former code bearer, it merely makes them mortal again. The nun was lying in a pool of blood because she committed suicide after regaining her mortality. V.V. was still alive for quite a while when Charles had his code.
Killing the former code bearer is NOT a requirement to get the code. C.C. herself had been severely injured by the nun (to force a dilemma on her "accept my code and live, or refuse and die") and was lying on the floor bleeding, she was in no shape to kill the nun. V.V. wasn't killed by Charles, he eventually succumbed to his injuries sustained in battle with Cornelia and Lelouch.
Codes serve as the connection to The World of C, and have enough "power" themselves to enact physical changes to the world ala Regeneration and their soul returning to their bodies. Geass is a manifestation of the users soul plus "power", the bridge between it and their codes getting larger as more "power" is transferred. Once it grows large enough it can "suck" the code through the link. This is probably accidental, with the original owners not wanting to die otherwise why would they seek immortality. The codes can do more than the automatic abilities looking at C.2's telepathy(?) but this knowledge was lost with their original bearers and has to be rediscovered on their own (thus why V.V can't).