Conjunctions are used to show a relationship of likeness or contrast between two words. Conjunctions join words, phrases, and whole sentences together.
By use of conjunctions, writers and speakers can connect clauses into more complicated phrases and sentences, making writing and speaking cleaner.
Many conjunctions are composed of only one word, as are the seven common coordinating conjunctions: for, and, nor, but, or, yet, so. Some conjunctions are phrases, such as the correlative pairs "not only/but also" and "just as/so too."
Coordinating conjunctions (for, and, or) and correlative pairs are used when two ideas are equally important. For instance in the sentence: "Both Sheila and I went to the store," "both" and "and" are a correlative pair. On their own, "but" and "and" are coordinating conjunctions.
Subordinating conjunctions make one idea less important than its complement, as in this sentence with the conjunction "before: "Before I went to the store, I took out the dog."
The use of several conjunctions is governed by standards of English writing. For instance, many believe it is not proper to begin sentences with either "and" or "but," and that beginning a sentence with "because" should be reserved for fronting the dependent clause in a complex sentence (i.e., "Because she is tall, John wanted to ask her to see a film.").