Conjunctions have two basic functions or "jobs":

Coordinating conjunctions are used to join two parts of a sentence that are grammatically equal. The two parts may be single words or clauses, for example:
Example:
1. Jack and Jill went up the hill.
2. The water was warm, but I didn't go swimming.

Subordinating conjunctions are used to join a subordinate dependent clause to a main clause, for example:
Example:
1. I went swimming although it was cold.