This is a topic that could easily take a couple articles to cover the bare essentials - but I'll try to see if I can summarize the key useful info.

It depends on a variety of factors, but probably the most important factors are the cut, the quality of the cut, and the temperature to which the cut has been cooked (in no particular order).

Cuts from the back are generally more tender because the muscles have been used less. A couple examples are: tenderloin (including the filet), top loin (such as New York strip), and rib (like a rib-eye or delmonico steak). A rib-eye steak has more intramuscular fat than a filet and this will also affect the tenderness and flavor. A filet (the choicest cuts from the tenderloin) is almost always the least used part of the least used muscle and is therefore very tender. However, it doesn't contain a lot of intramuscular fat, so it tends to have a less beefy flavor than other cuts. Another factor of having less fat is that when cooked to the point where the proteins tighten up, there isn't any fat to melt into the steak and lubricate it, so you lose a key factor in how tender the meat feels to your mouth when cooked beyond rare. The rib-eye has more intramuscular fat (the distribution of which is referred to as marbling) and so tends to have more flavor and a more tender consistency when overcooked (i.e. beyond medium-rare).

The quality of the meat is determined by three main factors - the age of the cattle when slaughtered, the amount and distribution of intramuscular fat, and the aging process of the meat. I will skip the first factor (almost all cuts you will be buying are from young cattle - older cattle produces meat for canning, hamburgers, and commercial meats) and briefly explain the other two. The marbling is important because of the affect of fat as it melts and spreads through the cut during the cooking process. It brings flavor and a perceived sense of tenderness. The more marbling, the higher the quality of beef and the higher the USDA grade (Prime has more marbling than Choice which in turn has more then Select). Be aware that many popular supermarkets have recently taken to branding their beef (usually USDA Select) to bolster sales of lower cost cuts at a higher price point. It's probably best to examine your steaks and look for ample amount of white flecks of fat dispersed liberally throughout the cut than follow fancy sounding supermarket labelling. The aging process also serves to tenderize beef. Generally supermarket beef is wet aged (sealed in a vacuum bag and refrigerated for a couple weeks) to allow the natural enzymes to begin decomposition of the tough proteins - naturally tenderizing the beef. In the U.S. almost all beef sold is wet aged to provide tender beef. Some places will dry age (sometimes they will wet age and follow it by dry aging) where the beef is not vacuum packed and is hung in a refrigerated compartment or room where humidity and temperature are controlled to allow the enzymes to do their work without the meat going bad. This is typically done for about 14 days for more supermarket dry aged beef (if you can find a market that carries it) to up to 1-2 months depending on how lucky you are and how much you are willing to pay. Dry aging yields a superior flavor (dry aging enhances the flavor of beef while wet aging does not) and tenderizes effectively. Unfortunately dry aging takes up space and results in a significant loss in edible beef as the exterior of the cut drys during aging and must be cut away by the butcher before sale. Therefore, dry aged beef is more expensive - but more flavorful and usually more tender than the standard wet aged beef.

Finally, we come to the cooking part. After selecting a cut that maximizes your chances of tenderness (let's say a USDA Prime filet or rib-eye that has been dry-aged for 3 weeks), you'll want to make sure you don't overcook it. An accurate and fast meat thermometer (such as a ThermaPen) is a vital tool for hitting the exact same temperature window every time because (unless you cook steaks everyday) it's difficult to tell how fast the temperature is rising in a steak as your cooking because, between two different meals, your steaks may be a different width or shape or the starting temperature may be drastically different... anyway, what temperature are we aiming for to achieve maximum tenderness? I'd say 125°F at the center. At this point, the proteins have just begun to tighten and form the tell tale striations that reveal that water has just begun to be squeezed out. Further cooking will cause the muscle fibers to continue to tighten and more and more water and juices will be lost. Cuts with more intramuscular fat will help mitigate the loss of tenderness. Most people's tastes tend to lean toward a more cooked steak at around 135-140&176;F - but the steak has become noticably less tender at this point - which is considered medium-rare to medium. Thicker steaks make it easier to sear the outside forming a brown crust while the inside doesn't overcook. Steaks thinner than 2-in. often have a tendency of being unpredictable and disappointing (either the outside is perfect and the inside overcooked or the inside perfect and the outside laking the rich, brown crust that is the only reason that makes a steak better than a slow roast).

To accomplish a 125°F, you should grill on the high heat. With a gas grill, that's basically as much heat as you can muster. On a charcoal grill, build a two level fire with a generous amount of lump charcoal. When the initial fire has burned down a bit, slap the steak down and leave it for five minutes. Using tongs, flip the steak over and leave it again for five minutes. Pick up the steak with tongs, insert the instant read thermometer from the side (through the part of the steak that is perpendicular to the grill) and measure the middle of the largest muscle of the steak (the coldest part). Take it off the grill to rest once it hits 110°F. If it hasn't hit 110°F, place the steak on the cooler part of the grill and check again at regular intervals (every 30 to 60 seconds). Keeping your steaks refrigerated until grill time also helps as extra insurance that you won't run past the designated temperature as you sear the outside. Cover the steak with a tent made of aluminum foil and wait ten to fifteen minutes. The unevenly heated steak (very hot on the outside, 110°F on the inside) will even out the temperature a bit, causing the interior to warm up to about 120-125°F - perfect.