When things get their name from something, with a difference of ending, they are called paronymous. Thus, for example, the grammarian gets his name from grammar, the brave get theirs from bravery.
This passage is better understood if we know that it is describing Greek. Greek languages, like some other languages, allow you to add endings to common nouns in order to change them. So we take a word like bravery, add an ending, and it now refers to people who are brave. Similarly, we can take the noun grammar, add an ending, and it now refers to people who teach and learn grammar.
The problem with these three categories of words is one of relevance. Why did Aristotle choose to open the Categories with a discussion on the nature of similar words? Later on, Aristotle will make use of these categories to show that a certain kind of word is always synonymous.
Next, Aristotle divides what is said into two categories.