According to the major league rulebook, section 10.15A (found on mlb.com), "a wild pitch shall be charged when a legally delivered ball is so high, or so wide, or so low that the catcher does not stop and control the ball by ordinary effort, thereby permitting a runner or runners to advance."
In other words, if the pitch is wild (duh) and a baserunner advances as a result, the pitcher is charged with a wild pitch.
The abbreviation for wild pitch is WP.
St. Louis Cardinals pitcher Rick Ankiel has become infamous for his lack of control, tying a major league record with 5 wild pitches in one inning in October 2000 in Game 1 of the first round of the playoffs against the Atlanta Braves (Bert Cunningham of Buffalo accomplished the same feat way back in 1890).
According to baseball-almanac.com, Nolan Ryan threw the most career wild pitches (with 277).