From BR Bullpen

Control refers to a pitcher's ability to throw the ball where he wants it to go.

Control is usually measured by a pitcher's walk rate, K/BB ratio or percentage of pitches that were strikes. These are imperfect measures for a few reasons:

1) They count intentional walks, which are not attributable to a pitcher's poor control. 2) The third measure discounts pitches thrown intentionally out of the strike zone, for instance on a 0-2 count to try to get a batter to swing. 3) They do not take into account other signs of poor control such as wild pitches or hit batsmen. Hit batsmen, though, tend to reflect pitchers who work inside or who are willing to throw a beanball in addition to those with poor control.

Notable control pitchers have included Babe Adams, Greg Maddux, Christy Mathewson, Deacon Phillippe, Sam Leever, Robin Roberts, Ferguson Jenkins, Cliff Lee and Bill Fischer.