A conjunction joins words, phrases, or clauses, and indicates the relationship between the elements joined. Coordinating conjunctions connect grammatically equal elements: and, but, or, nor, for, so, yet. Subordinating conjunctions connect clauses that are not equal: because, although, while, since, etc. There are other types of conjunctions as well.
"The young girl brought me a very long letter from the teacher, and then she quickly disappeared."