Categorization is one of the first things we do when we begin to attempt to produce a body of knowledge. That is we arrange the things we know by means of abstract labels that are systematically related to each other. However knowledge is more than merely a set of categories with various things assigned to them. For one thing if you look into Systematics and Ecology is that different category schemes reveal different things about the categorized items. And thus we really need multiple categories to express how things like species are related to each other. Depending on how you produce the category scheme things they may fall in different categories.
But beyond that we might derive some deeper understanding by seeing that there is a hierarchy of emergent levels that goes from data to information to knowledge to wisdom to insight to realization. Now categorization is a way of relating bits of data to each other so it is essentially at the level of information. It is not really knowledge because of the fact that different categories will cause different data items to show up in different relations to each other in the various category schemes. Knowledge is normally more than information, and is usually something about the way things fit together and work together within the world. Newton’s formula for the laws of motion, and the realization that the planets and things on earth obey the same laws of motion are usually cited as the case in point. At that point we had a fundamental understanding of the phenomena of motion and its relation to the force of gravity. That was knowledge and the same equations are used today. Knowledge has a kind of permanence that few other things have in our experience. A body of knowledge should be a listing of those things that are stable in our experience once we understand them. Categories change and are fitted to the situation of their use. Knowledge changes but normally by paradigm shifts, episteme changes (i.e. changes in fundamental categories by which all experience is understood) and by crucial experiments being performed, or by the discovery of new phenomena. We don’t have a good term for a unit of knowledge. It has been called a “knowl”. But normally we think about a network of concepts that form a theory, and the facts that are covered by that theory.
So in answer to your question I would say that categories are the threshold of knowledge where information gets sorted so that we can begin to try to get a deeper understanding of if which we express as theories which are tested, etc.