A marinade is an acidic liquid and/or brine that penetrates the beef, giving it flavor and breaking down its muscular interconnectivity. That is, it makes your beef tender and flavorful. Marinades consist of vinegars, wines, citrus, or soy sauce to name a few. You can marinade an hour or overnight (in the fridge). Large zip lock bags help.

I've been doing a lot of reading and experimenting with simple marinades as of late. My latest resource is the Cook's Illustrated Guide to Grilling and Barbecue. It comes highly recommended. The key is a salty marinade and the best is soy sauce. Start with soy sauce as a base, then add vegetable or canola oil, enough garlic and chili powder, cumin, and cayenne to taste and you've got a southwestern marinade to die for. Pears, mangos, and pineapples have natural enzymes that assist the tenderizing process for a fruity twist. Another great tip is productive (ie soy sauce based) marinades really only need 45-60 minutes. Overnight and it becomes mushy. Lastly, leave the citrus out of marinades. It just doesn't help. Left too long and it will make your beef tough! To best capitalize on that citrus goodness, use a post-marinade. After taking beef off the heat, beef will reabsorb some of its juices, including any flavors you impart on the surface. For example, with my prior southwestern suggestion, save about 1/4 cup of marinade per pound of beef and add the juice of half a lime. Once your beef comes off the heat, bath in the citrus post-marinade, letting rest for ten minutes. Yum!  

Rubs are a mix of spices and/or herbs that are rubbed on the meat. This creates a spicy shell when cooked. The flavors, if allowed time, will penetrate the beef creating interesting flavors. Use your hands to literally rub the mixture into the surface of the beef. Rubs are either wet or dry, the only distinction being whether you use olive oil or not. If your rub has salt in it, and pretty much all of my rubs do, it is best to let it sit for 30-45 minutes or so, ideally wrapped in plastic wrap. The salt will penetrate the beef and assist in tenderizing. I like rubs. For ideas, mix and match the following: