First, let's get our terminology right. There are 4 "genders" of beef:
Cow: this is the mature female who has had 1 or more calves. She can have 1 calf per year, and usually she is bred for 6 to 7 years, although 10 years isn't impossible.
Bull (photo to the right): this is the mature male with his reproductive organs intact. A farm usually only has 1 bull and he can "service" up to 75 females in as short as 3 month time window.
Heifer: this is the unbred female
Steer: this is the neutered male
Generally, the quality of beef is related to the genetics, the management of the herd (grazing practices), the feed (quality of the pasture), the age, and the gender.
Fat is important in beef for tenderness and flavor. Fat comes from the quality of the feed and the gender. A bull is generally very muscular and very lean; a cow will tend to be fattier, but her condition varies a lot depending on how recently she carried a calf.
Tenderness also comes from the age of the animals. Over 24 months and the meat structure becomes more firm and less tender. As a result, we tend to slaughter beef that is young and gender neutral - generally steers and heifers. Cows and bulls are generally used to grind for burger, sausage, or cooked products like bologna.
Finally, if you look at gender and age together, you realize that the male hormones create lean beef. It is common in many beef operations to "steer" the bull calves when they hit 9 months of age. During those first 9 months, the bull calf grows faster with the testosterone. However, I "steer" my bulls at day 1 so that they grow slower and develop more intramuscular fat as opposed to lean muscle. In a traditional corn-finished feedlot operation, the high energy grain diet is used to equal things out at the end.