Beef and lamb are pretty much loaded with all the fat and moisture you'd ever need.
On the other hand, chicken, turkey, and pork sometimes require some assistance. It's important when you've got a piece of meat that has a tendency to get overcooked and thus dry and might benefit from a little sugar and salt. This is vitally important for chicken and turkey, and apparently benefits pork loin, tenderloin, chops, and fresh ham, shrimp, and whole sides of salmon.
You want to be careful, of course. Butterball and Kosher turkeys, for example, have been treated similarly to brining already. It tastes even better if you do it yourself, but keep an eye on if the meat has been "koshered" or "injected" beforehand.
So, how do you brine? Well, I'm using the formulation given by Cooks Illustrated some years ago and they've worked just fine for me.
My usual formula is 1 pint of water with 1/4 cup Diamond Crystal Kosher and somewhere under 1/4 cup sugar. You want to use enough brine to cover the meat totally, but enough that you don't run out of brine (If under 2 gallons, at least 1 qt per piece-pound). And you want to do it for at least 1 hour per piece-pound (meaning if you have two 4 lb chickens, 4 hours, but if you have two 4lb chickens cut into pieces, more like an hour) but not more than 8 hours. If you are doing something with the skin on, then you can air-dry it in the fridge for a few hours, maybe even overnight, to make the skin nice and crisp again.
Here's the complete table:
|Water||1 qt||1 qt|
|Salt||1/2 cup Diamond Kosher / 1/4 cup + 2 tbsp Morton Kosher / 1/4 cup table||1/4 cup Diamond Kosher / 3 tbsp Morton Kosher / 2 tbsp table|
|Sugar||1/2 cup||2 tbsp|
The science behind it is that all of the cells that make up your meat have membranes. Water will diffuse across the membrane from a point of higher salinity to lower salinity, carrying a little bit of the salt and sugar across. This means that you don't get the incredibly salty piece of meat you might expect by leaving it sitting in concentrated saltwater, but instead you get a lot of yummy moisture.
It also carries anything else water soluble across the barrier. This means that you can improve a marinade by making it grievously salty... although if you are using part of the marinade to baste it, you'd better add the salt after separating that part out.