Normally meat loses about 30 percent of its weight during cooking. This is because as meat cooks the fibers bind to each other more tightly, which forces out moisture. Also, as the meat cooks and the proteins bunch up the meat shrinks. That's why it's important to rest meat before you serve it. It gives the protein time to relax and the juices flow back into the meat. You can exert some control on this process by brining the meat.
Here is what happens: The salt in the brine causes some of the proteins to break down, or denature. As the proteins denature the salt binds directly to them, which prevents the proteins from binding to each other when they cook. As the meat cooks and the muscle fibers tense up they can't grab other muscle fibers and wring out the moisture. The end result is a jucier steak, pork chop, chicken breast or piece of fish.
Did I just say fish? Why yes I did, because there is also an aesthetic reason for brining. Have you ever noticed when you cook meat -- but especially fish -- that some weird looking foamy white stuff comes out and looks like egg whites when it cooks? That's because it's the same protein that's in egg whites. As the muscle tenses up during cooking the albumin liquefies and gets forced out. This is the protein that brining breaks down. So if you brine your meats first this protein won't ooze out, giving you a nicer-looking end product.
Below is a chart showing some general guidelines for brining different meats.
I also brine steaks and roasts in a 10% by weight brine for 24 - 72 hours. That is 3.5oz salt to 1QT (32oz) water. (Technically it's 3.555 oz, I round down to make it easier on me. And yes, I use a scale.) You can add other ingredients to further enhance the flavor, such as garlic powder, onion powder, cardamom, cloves, tamarind powder, cayenne, preserved lemon, or just about any other spice. Just be careful about adding too much. Since it will get soaked into the meat you can end up with a very oddly flavored end result.
I find that meat brined in a lower sodium brine for longer is more tender and flavorful when cooked, but that's simply my observation. Your mileage may vary.
Image from http://www.finecooking.com/articles/why-brining-keeps-meat-moist.aspx. Check out that article for more information and tips on brining.