POS is concerned with identifying the types of components found in a sentence. For example, this sentence has several elements, including the verb "has", several nouns such as "example" and "elements", and adjectives such as "several". Tagging, or more specifically POS tagging, is the process of associating element types to words.
POS tagging is useful as it adds more information about the sentence. We can ascertain the relationship between words and often their relative importance. The results of tagging are often used in later processing steps.
This task can be difficult as we are unable to rely upon a simple dictionary of words to determine their type. For example, the word lead can be used as both a noun and as a verb. We might use it in either of the following two sentences:
He took the lead in the play. Lead the way!
POS tagging will attempt to associate the proper label to each word of a sentence.