I am not a professional, so please do not read any further.
In the USA, a copyright is automatically held by whoever created whatever was created from the moment that it was created. If you grant rites to someone else, such as the person who hired you to do a certain job, they can use your work. In fact, they can claim the copyright if you created the work while employed by or under contract with a third party. The US Copyright office says, "In the case of works made for hire, the employer and not the employee is considered to be the author," and can claim the copyright.
It would be worth going to http://www.copyright.gov/circs/circ1.html#wccc and clicking on "Who Can Claim Copyright," as this gets rather tricky and detailed.
Can they sell it without your permission? Let a pro tell you that. It is my opinion that they can USE it in reports or promotions, etc., but that they can not ethically sell it without your permission. The law reads as if they can, though, so this should be stipulated in a written agreement. Most of the time, this would not be an issue. Okay, if it was the next Farah Fawcett poster, I guess the model would want to sell it. You would have to have a model release to sell it or she would have to have your permission to sell it (in my opinion) unnless she paid you specifically to take her photos to be used for resale, so these things have to be worked out. You can still own the copyright and give permission to the other party to use your work, even if you were not paid. It's hardly worth mentioning, but I have several photos on my city's website. I own the photos and they are using them with my permission. I do not fear that they will use them inappropriately, because I am the webmaster's dentist and I can hurt him if he screws me.
If you want to ENFORCE copyright ownership, you would be smart to register the piece in question with the US Copyright Office. All this really amounts to is thrid party verification of the date of creation. If you want to sue someone, it is easier to produce a copy of your copyright registration to prove that the work is yours and it was done on such-and-such a date than it is to prove this by other means. In photography, it used to be pretty easy to prove ownership with possession of the negative. In the digital world, it is much easier for someone to steal your work. I like to use Flickr, for instance, but I realize that I might get ripped off. If I made a living this way, I doubt I'd let people view my work full-sized. As it is, much of what I do there is for illustrative purposes to support an answer over here and if I disallow downloading the original size, people can only view about a 500 pixel wide image and this would not show the differences in sensor resolution or how crappy digital zoom is, etc.
I've had experience in registering a copyright for a book and for music, where thievery would be harder to prove without the copyright. Who's to say I didn't steal from the person who actually stole from me? This is much harder to prove.
See http://www.copyright.gov/ for some thoughts on this. WOW! I see the fee has gone up to $45! I paid $10 not that long ago.
Didn't I tell you to ignore my answer? Go on now.
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ADDITIONAL ANSWER
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Copyright issues are something that I always caution young photogrphers to be careful with when they are considering whether or not to enter a contest. Some let you keep the copyright and give them the right to use your photo in publications or promotions. Some claim the winner's copyright and state that they will be able to use the image in promotions OR FOR PROFIT. This simply is not right, in my opinion. It's usually spelled out in a 3-point font that nobody bothers to read, but by entering the contest they agree to those terms.
Please also see my additional comments under "Source" below.
And thank you for not being offended by my attempt at an answer, even though I am not a profesional photographer, the way some people (person) was last week.