Brining is a broad description for the many various ways to salt meat. Generally, it's done to lighter and/or leaner meats. It's primary function is to evenly distribute salt within the meat. All you need to brine is salt & water.
That being said, brining also changes the cell structure of the meat. It tenderizes and moisturizes, as well as imparting flavor. You can brine any meat you will be cooking, but it lends itself well to long, slow cooking processes that would otherwise dry out a piece of meat. But it also helps ensure even cooking (especially in poultry).
A brine isn't water with salt in it. It's generally going to be close to the saturation point, so you'll have to heat the water in order to get the salt to dissolve. Personally, my salt threshold is 1C salt to 5C water. But you then have to chill the water back down to submerge your meat. I pour the mix over about 2C of ice, so my brine ends up being about a 1:6 ratio. Imagine the saltiness of sea water....that's about right. If it tastes "good" like a well seasoned soup, it's not enough salt for a brine.
While the water is simmering up to temperature, it's a good time to add your flavors. Aromatic herbs (thyme, rosemary, oregano, etc.) go well, definitely garlic, some onion, and it always helps to add about 1/2 C of brown sugar. This is where you can play with the flavors and combinations.
