Copyright ownership depends on the client and work situation, but should always be formalized in a contract or letter of agreement. Your compensation should be proportional in some way to the client/employer's usage/benefit of the work.
If you're an employee, your employer usually owns everything you do (in some cases even outside of work hours). You must ask permission to use any work, and often to take on any work "on the side". The premium paid for this is job security.
If you're a freelancer, it's a little more complicated but again should be formalized in a contract.
Periodicals (magazines, newspapers, buyer's guides, etc) and ad agencies usually only need partial copyright. Their usage may be limited by time, circulation/print run, geographical region, medium, etc. The premium here isn't paid by the commissioning client, but through resale to other clients.
Corporate clients (manufacturers, engineers, designers, marketing divisions) and pretty much everyone else will usually want full rights, exclusivity, and sometimes non-disclosure. They'll want to use the image everywhere they can, be the only ones using it, and sometimes prohibit you from using it. A premium should be paid since they're getting so much mileage out of it, you can't resell it, and you can't show it to prospective clients to get more work.
And when I say "premium" here, I just mean you should be working something into your compensation so that what you get from the client is in some way proportionate to what they get from you. You shouldn't separate it out on an invoice, just consider it when setting your prices.
The
Graphic Artists Guild Handbook covers all of this in much more detail.