From BR Bullpen

A no-decision refers to a game in which the starting pitcher is credited with neither a win nor a loss. This may be the result of his leaving the game with the score tied, of the outcome of the game changing after his departure from the scene, or of his failing to complete five innings in a game his team is leading and eventually wins.

Once upon a time, there was a stigma attached to a no-decision, implying that the starting pitcher had failed in his duty to pitch until the outcome of the game was decided. Nowadays, this no longer applies. Pitcher usage is dictated more by the pitch count than by the score, and measures such as game score or quality starts were devised to gauge a starting pitcher's effectiveness independent of his won-loss record.