A bull is a male cow. You can usually tell which animal in a pasture is a bull by its large size and horns.
A male bovine — or cow — is a bull, and so is a male whale or elephant. Fittingly, the word bull is sometimes also used for a particularly bulky, muscular man. Another informal and slightly obscene meaning is "ridiculous," or "not true," as when you tell a lie and your brother rudely replies, "Bull!" This slang meaning has its roots in the Old French bole, "deception or trick."