What does brining do?
Brining is the soaking of meat in a solution of water and salt. Additional flavorings like sugar and spices can also me added, but salt is what makes a brine a brine (just like acid makes a marinade a marinade). This soaking causes the meat to gain some saltiness and flavoring while plumping it up with water so that after cooking it still contains a lot of juices.
Obviously, there's more going on than simple osmosis. It is true that salt enters the meat (it tastes more salty after brining). But why is it also more juicy? Well, when water flows out of the meat, salt flows in and begins to break down some of the proteins in the cells. In the broken down state, the molecules become more concentrated and the solute levels rise within the meat. This causes additional water to flow into the meat.
But doesn't that mean we've got the same amount of water as before brining? Nope. The cell membranes are semipermeable. They allow salt and water to flow in both directions freely, but larger molecules (like the denatured proteins and other solutes in the meat released by the salt) cannot flow out from within the cells. When the solutes of a solution on one side of a semipermeable membrane cannot pass to the other side, osmosis causes more and more solvent to move through the semipermeable membrane. This continues until the extra pressure from holding more solvent equals the rate at which solvent is "drawn" through the semipermeable membrane. (This rate is called osmotic pressure. How Stuff Works has a short article describing osmotic pressure with a diagram that may be helpful to visualize the water flow.)
What has happened is that through brining, we've caused a state change in the cells so that they will draw and hold more water than before. As we cook the meat, the heated proteins will begin to draw in tighter and squeeze out water, but, hopefully, enough water will remain to produce a juicy, tender piece of meat.
So, how much salt in water is used for brining? That really depends on how long of a brine you want and how salty you want the final product. A weak brine will require a longer brining time to achieve the same saltiness as a strong brine. When I need a moderate strength brine, I use 1/2 cup (about 150 g) of table salt per gallon of water (40 g per liter or 4% salt brine). (Higher concentrations of salt can be used to reduce brining times, but the amount of salt and the time it takes to brine is dependent on the muscle structure of the particular piece of meat.) Using kosher salt is a common practice, but different manufacturers grind the salt to different levels of coarseness, so kosher salt should be weighed before adding to water. For small amounts of salt, the salt can be dissolved into cold water, but for larger quantities it may be necessary to heat the water to dissolve the salt.
Always start with a cold brine. If you heated the brine, then refrigerate it before using it. The raw meat will be in the brine for a number of hours, so we don't want the temperature of the meat to rise higher than refrigerator temperatures (40°F, 4°C) if we can help it. Place the brine in a noncorrosive container like a plastic or glass container, plastic bag, or a stainless steel pot.
The brining time depends on the shape of your meat as well as the type of meat. Generally, a good rule of thumb is 2 hours per pound of solid poultry when using the 1/2 cup salt per gallon (40 g salt per liter; 4%) brine. Cut up poultry will have reduced brining time. For chicken pieces like breasts or thighs, 2 hours is usually enough time. Pork may take about four times as long to brine as poultry. In most cases, it's difficult to predict how fast the salt moves into the meat when you double or halve the salt in the brine, but it's worth experimenting with to have your brining "finish" at a time where you will be around to remove the meat from the brine.
When you remove the meat from the brine, rinse off the excess salt from the surface and return the meat to the refrigerator to await cooking. Pour out the brine after each brining. (No need to have a half gallon of raw meat juice infused salt water lying around growing germs...)