The grammar of a language is an analysis of the various functions performed by the words of the language, as they are used by native speakers and writers.

There are many different ways of analyzing a language. In such an analysis, words can be given various names, depending on the function which they perform. For instance, words which perform the function of naming things are commonly referred to as nouns, and words which perform the function of expressing states or actions are commonly referred to as verbs.

It should be kept in mind that many English words can perform more than one function. For instance, in the following sentences, the underlined words can be referred to as nouns because they perform the function of naming things.
e.g. I have lost my comb.
      Water is one of the necessities of life.

However, in the following sentences, the same words can be referred to as verbs because they perform the function of expressing actions.
e.g. I comb my hair every morning.
      Do you water your plants once a week?