In a sense, one could answer this by saying that while Plato struggled to define "the good", Aristotle resolved the problem by saying that "the good" in and of itself does not exist.
The essential distinction has to do with Plato being concerned about the Forms, the notion of a deeper reality of abstract ideals of which the phenomena we perceive with our senses are merely faint shadows or imitations. For Plato, all things we consider good participate in the Form of the Good.
For Aristotle, "separated" forms did not exist. Goodness was simply the commonality we perceive among things we consider good and of which we approve. In his ethical works, he suggests that we are naturally drawn to good things and thus we use the term goodness to describe them. For Aristotle, the highest human goods are those we seek for themselves alone, rather than as means to something else. Thus, he would say that the highest human good or type of well-being ("eudaimonia") is the virtuous life.
In technical terms, Plato is a "realist", one who considers that abstractions possess some type of independent reality, while Aristotle is a "nominalist" who argues that our abstract terms are merely shorthands for perceived commonalities and have no independent existence.