A part of speech is one of the nine types of English words: VERB, NOUN, ADJECTIVE, ADVERB, PRONOUN, PREPOSITION, DETERMINER, CONJUNCTION, INTERJECTION. There are thousands of words but they don't all have the same job. For example:

1. verb (deliver - expresses action)
2. noun (computer - expresses a thing)
3. adjective (yellow - tells us more about a noun)
4. adverb (quickly - tells us more about a verb)
5. pronoun (it - replaces a noun)
6. preposition (on - links a noun to another word)
7. determiner (the - limits a noun)
8. conjunction (and - joins words)
9. interjection (ouch! - expresses feeling)