Full Idea
Knowing and understanding is of two kinds, one having and the other using knowledge.
Gist of Idea
Knowing is having knowledge; understanding is using knowledge
Source
Aristotle (Eudemian Ethics [c.333 BCE], 1225b11)
Book Reference
Aristotle: 'Eudemian Ethics I,II and VIII', ed/tr. Woods,Michael [OUP 1992], p.28
A Reaction
This corresponds to potential and actual. We wouldn't say that understanding must be used, but we have some sort of distinction between knowledge as pure and theoretical, and understanding enabling good application.