A conjunction is a  part of speech that is used to connect words, phrases, or sentences. 

Examples in English include: but, and, as, because, so, yet etc.

Conjunctions connect thoughts, actions, and ideas as well as nouns, phrases and other parts of speech, e.g., John came to work early and finished late.

They are useful for making lists, e.g., I would like juice, coffee and a muffin please.

Conjunctions must agree, e.g., Sarah was tired yet she went to sleep, does not show agreement. Sarah was tired so she went to sleep, does show agreement.