Sink drains aren't usually glued in. They sometimes have silicone, but often they are held on by pressure and plumber's putty. Some may even have rubber gaskets instead.
As sheplmbr said, there should be a nut underneath keeping the sink drain attached.
As to the discoloration, when certain metals touch one another, it causes a chemical reaction which can cause corrosion. That is why you should use products that use the same metals and also try to avoid getting ones that have ferrous metal (iron) because they are prone to rust.
Since it appears you have a garbage disposal, it might be a little bit trickier to remove the drain flange. You will have to disconnect the top of the garbage disposal unit from the bottom of the sink. Once there is no longer pressure pulling the flange down, you can push up on the drain/flange to pop it up. If you see any old plumbing putty, wipe it off.
Hopefully this video will be helpful. I couldn't load it due to bandwidth restrictions.
[ame]https://www.youtube.com/watch?v=mhAzDp4M_cI[/ame]
There is also this article:
http://www.ehow.com/how_7020025_replace-kitchen-sink-flange.html
I hope this helps.
Edit: I didn't see Frodo's post before I posted this.
I had to change a garbage disposal out before so I'm trying to remember the steps.
I think you have to remove the disposal to access the screws that hold the flange on. You can set a bucket or something underneath the disposal to hold it up.
