Save percentage represents the percent of time a pitcher records a save when given a save opportunity. Obviously, then, save percentage is calculated by dividing a pitcher's total number of saves by his total number of save opportunities.
Save percentage can be a useful tool for evaluating pitchers who are strictly closers. Ranking pitchers by total saves isn't effective because saves are so reliant on outside factors that a closer can't control -- such as how often a closer's team has a lead in the eighth or ninth innings. But save percentage levels the playing field between closers who get many save opportunities and closers who don't. It simply answers the question: How successful is a closer at getting a save when he has a chance?
If a reliever records a hold, it does not affect his save percentage, because he has not been credited with either a save or a blown save.
The term save was used by general managers in the 1950s, without specific parameters. It simply referred to a relief pitcher who entered the game with a lead and finished off a win -- regardless of score. Writer Jerome Holtzman was the first to give specific criteria to saves in the early 1960s. But saves didn't become an official stat until 1969.