A conjunction is a word that joins two words or group of words.
There are two types of conjunctions - coordinate or subordinate.
These are used to join two parts of a sentence that are grammatically equal (that is two statements are of equal importance). The two parts may be single words or clauses, for example: Jack and Jill went up the hill. The water was warm, but I didn't go swimming.
These are used to join a subordinate dependent clause to a main clause, for example: I went swimming although it was cold.