What’s the difference between assigning your copyright and licensing it? While they sound like they might mean the same thing, they’re actually quite different legally speaking.
And before I go on, my usual disclaimer applies: Nothing written here is intended as formal legal advice and folks who need help with particular issues should consult an agent or attorney.
As you probably know, a copyright is a legal property right in the expression of your work (the actual words of your story or article). The right is created by Congress in the Copyright Act and attaches to your work as soon as you write it, or in “copyright-speak”, as soon as you fix the work in a tangible medium of expression, like on paper or in a digital file.
Like any property right, your copyright can be assigned or licensed. An assignment is basically a complete sale or transfer of the right, and a license is a permission for someone else to use the right on the terms set out in the agreement. In many ways it really is just like any other property. You can buy a house to live in (outright sale and you own the house one hundred per cent, maybe subject to a mortgage) or you can rent a house (the landlord continues to own it, but gives you the right to live in it subject to payment of your rent).
The same applies to your copyright. You can sell it to another person (publisher) and then they will own it one hundred per cent and you will have no further rights in it. Or you can license it and they can pay you “rent” which, in the copyright context, usually means “royalties” i.e. a fixed percentage on the copies of your work that they sell.
A license usually gives you more rights in your own work because you have not given up all your rights: you’ve simply given the publisher permission to market the work under the terms of the 
license. The terms of the license can be limited to a particular time period, a particular geographic market (e.g. North American publication rights) or a particular version of the work (e.g. electronic publication).
Most commercial publishing contracts for books and magazines ask for “first sale rights” which are usually a license to the publisher to publish the work first in a particular market (e.g. North America, the whole world, etc.), but the author retains the copyright. These licenses may or may not be “exclusive” in the sense that the publisher is the ONLY person who can market the work for a particular period of time.
However, if you’re a freelance writer or otherwise writing “works for hire” you may be effectively granting or assigning your copyright directly to the publisher so that you have no rights left.
It’s important to understand the difference between assignments and licenses and, if you’re writing for someone else or selling your work to someone else, to understand exactly what rights you’re keeping and what rights you’re giving away. Assignments and licenses are simply ways of entering into contracts involving your copyrights, so make sure you negotiate and understand an agreement that works for you. Sometimes you don’t have a lot of leverage in a contract situation, and you have to decide whether to take it or leave it. Even in these situations, it’s important to understand what you’re being asked to give away and whether those terms work for you.