In this country, it's the Agricultural Marketing Service (AMS)—an agency within the U.S. Department of Agriculture (USDA)—that has a decisive say in how meat is labeled and marketed for sale. The AMS's programs include basic standards for meat quality (prime, choice, etc.), along with regulations for the certification of a whole slew of terms you've probably spotted on your packaged steak and burger meat—think antibiotics, hormones, and what exactly constitutes grass feeding.
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Here are the different classes of USDA graded beef, and their characteristics:
Prime
This is your highest of high-end beef, and accounted for just under four percent of all graded beef in the United States in 2013, according to the USDA. Prime beef has a marbling score of "slightly abundant" to "abundant," meaning that there's a whole lot of veiny, delicious fat worming its way throughout the muscle. Mark Lobel, of Lobel's butcher shop in New York, says that "marbling [is] what makes the whole experience of eating steak delicious," adding that in Prime beef, Lobel's looks for "streaks that aren't heavy or clumpy. If you take a sharp pencil and you make lines going through, it should be that fine." Other indicators of high quality include a pinkish-red flesh and milky white fat, which indicate younger, healthier beef—a steer's meat tends to get darker and coarser the older it is at slaughter, and the fat starts to take on a grayish tinge.
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Choice

Making up the majority of USDA graded beef (around 66%), Choice beef contains "small" to "moderate" marbling. While there are plenty of perfectly good steaks within this category, especially with naturally tender cuts like tenderloin, it does encompass a wide range of quality. On one end of the spectrum, a Choice steak may have an equivalent degree of marbling to a Prime steak, but because of the older age of the animal at slaughter, it missed the higher grade. On the other, a Choice steak could be taken from a tender, younger animal, but have only a modest amount of fat and marbling.

The Choice category is where you've really got to be vigilant in order to get the most bang for your buck. For steak cuts, look for choice beef that still has a good degree of fine marbling throughout—this is what's called "High Choice." The USDA does oversee a number of certified programs for cattle breeds such as Black Angus and Akaushi that generally require the beef to have a marbling score in the upper levels of Choice or higher, so it may be worth seeking out beef labeled as such to get a better quality cut within this grade.
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