Brine is a liquid solution that contains equal parts salt and sugar or salt and spices. For instance, pickling is a form of brine that preserves vegetables, meats, and eggs. I don't consider brining a form of marinating, whereas some people do. To be considered marinating, I feel additional ingredients are necessary to infuse or break down the protein — think of prepping chicken in a mixture of spices with acids (beer) and sauce (soy, bbq, honey) before tossing it on the grill. But that's my opinion! Brining helps tough cuts of meat or large game birds to tenderize and retain flavor. Thanks again— salt, or equal parts salt and sugar.
Most home chefs brine on Thanksgiving or Christmas. But brine anytime. There's no hard rule. Once you continue to make brines or start making ways to infuse flavor, it becomes easy. That's how pit masters are born!
My favorite brines include citrus, rosemary twigs, thyme, cinnamon, anise, whole peppercorns, salt, and sugar. I don't recommend brown sugar in brine because the molasses in the sugar will cause the meat to caramelize and burn faster. It is possible, just be careful of the molasses and nurse it or balance the brine recipe.