Marinating involves letting a piece of food to stand, usually refrigerated, in a highly flavorful broth or sauce to add flavor. Many marinades have vinegar or some other acid.
Contrary to popular belief, acids in marinades will not tenderize meat! Any liquid that has a pH of 1 or lower will, but the only examples are stomach acid and battery acid, which are not very good to cook with. Most vinegars have a pH of 3, and the lowest pH liquid that is used for culinary use is citrus juice, which has a pH of 2, still not enough to tenderize. So those people out there that still believe that acids tenderize, go get some battery or stomach acid for your "tenderizing".
If you're wondering what the difference between marinades and brines is, here it is: Brines are basically salty water with other flavorings added. Marinades usually don't contain the levels of salt that a brine does, and most marinades don't have added water. They also contain acids.