(4) She has no ____.
(5) She can ____.
The first sentence can be completed with words like bicycle, shoes, worries, ability, home, etc., that is, with nouns, but not with words like went, happy, in, or cheerfully (verb, adjective, preposition, and adverb respectively). The second sentence works with words such as hide, fly, delay, lie, cry, etc. (verbs), but not shoes, beautiful, happily, into, etc. Such sentence frames show that there's more to a word than its meaning. Words also belong to categories, and knowing membership in a particular category lets us predict where the word can fit in the sentence.
(6) *My sons both graduation high school.
A sentence like (6) is ungrammatical because the slot that graduation occupies in the sentence requires a verb, not because the general meaning of graduation is inaccurate.
Some linguists avoid the term parts of speech and prefer to speak simply of categories. What is gained by changing the terminology?
It is true that "parts of speech" is misleading if we take the expression literally, as components of language. Clearly, there are many more parts to language than word categories. On the other hand, "part of speech", as a term of art, differs little in its basic meaning from category.
It's really the implication of the term--its association with old grammar books--that causes some to avoid it. I, however, find it hard to see enough difference between the two terms to justify abandoning so familiar a term as "part of speech." Although traditional definitions are muddled, in practical terms, older grammarians meant largely the same thing as modern linguists do with major categories such as noun, verb, or adjective. Even where old fashioned grammarians could not explain the parts of speech adequately, they would still assign the majority of words to the same categories linguists do. (The exceptions, we will see shortly.) In other words, even if traditional grammarians did not define what they were doing very well, their intuitions about these categories led them to many of the same conclusions. So the lexical categories are essentially the same thing as the parts of speech. The fact that the details differ doesn't really affect that essential similarity.
The insistence upon the generic term category, however, does have the virtue of emphasizing just what the parts of speech are, something that is opaque in the traditional term. For that reason, we will use part of speech and category interchangeably, keeping in mind that using the traditional term does not imply we accept the details of traditional classification uncritically. Instead, we will examine how these categories can be redefined to better reflect the way they actually work.