The major difference between simple, complex and compound sentences is that the first has only one complete clause while the latter have two or more clauses. While a compound sentence is formed by two complete simple sentences connected by a coordinating conjunction, a complex sentence results from an independent clause joined with one or more dependent clauses.
A simple sentence, synonymous with the term independent clause, minimally consists of a subject and a verb that create a complete thought. The simple sentence, "The bus is late," for example, contains the verb "is" and the subject, "The bus." This type of sentence can optionally include compounds of the two basic components as well as prepositional phrases.
In compound sentences, two simple sentences, such as "The bus is late" and "I must wait" are joined by one of seven appropriate conjunctions, in this case "so," resulting in "The bus is late, so I must wait." The other six coordinating conjunctions are "for," "and," "nor," "but," "or" and "yet." The majority of these conjunctions indicate a form of relationship between the two clauses.
Similarly to compound sentences, complex sentence are made of multiple clauses. A dependent clause, which lacks components necessary to be a complete sentence, is related to an independent clause by one of several types of subordinating conjunctions, such as "after," "if" and "whereas."