The Compound Sentence A compound sentence consists of two or more independent clauses (or simple sentences) joined by co-ordinating conjunctions like "and," "but," and "or": Simple Canada is a rich country. Simple Still, it has many poor people. Compound Canada is a rich country, but still it has many poor people. Compound sentences are very natural for English speakers -- small children learn to use them early on to connect their ideas and to avoid pausing (and allowing an adult to interrupt): Today at school Mr. Moore brought in his pet rabbit, and he showed it to the class, and I got to pet it, and Kate held it, and we coloured pictures of it, and it ate part of my carrot at lunch, and ... Of course, this is an extreme example, but if you over-use compound sentences in written work, your writing might seem immature. A compound sentence is most effective when you use it to create a sense of balance or contrast between two (or more) equally-important pieces of information: Montéal has better clubs, but Toronto has better cinemas. Special Cases of Compound Sentences There are two special types of compound sentences which you might want to note. First, rather than joining two simple sentences together, a co-ordinating conjunction sometimes joins two complex sentences, or one simple sentence and one complex sentence. In this case, the sentence is called a compound-complex sentence: compound-complex The package arrived in the morning, but the courier left before I could check the contents. The second special case involves punctuation. It is possible to join two originally separate sentences into a compound sentence using a semicolon instead of a co-ordinating conjunction: Sir John A. Macdonald had a serious drinking problem; when sober, however, he could be a formidable foe in the House of Commons. Usually, a conjunctive adverb like "however" or "consequently" will appear near the beginning of the second part, but it is not required: The sun rises in the east; it sets in the west.

The Complex Sentence A complex sentence contains one independent clause and at least one dependent clause. Unlike a compound sentence, however, a complex sentence contains clauses which are not equal. Consider the following examples: Simple My friend invited me to a party. I do not want to go. Compound My friend invited me to a party, but I do not want to go. Complex Although my friend invited me to a party, I do not want to go. In the first example, there are two separate simple sentences: "My friend invited me to a party" and "I do not want to go." The second example joins them together into a single sentence with the co-ordinating conjunction "but," but both parts could still stand as independent sentences -- they are entirely equal, and the reader cannot tell which is most important. In the third example, however, the sentence has changed quite a bit: the first clause, "Although my friend invited me to a party," has become incomplete, or a dependent clause. A complex sentence is very different from a simple sentence or a compound sentence because it makes clear which ideas are most important. When you write My friend invited me to a party. I do not want to go. or even My friend invited me to a party, but I do not want to go. The reader will have trouble knowing which piece of information is most important to you. When you write the subordinating conjunction "although" at the beginning of the first clause, however, you make it clear that the fact that your friend invited you is less important than, or subordinate, to the fact that you do not want to go. http://www.arts.uottawa.ca/writcent/hypergrammar/sntstrct.html

thanks It 's very useful

complex sentences are more complex :P

and here some useful links http://www.mccd.edu/faculty/pirov/compound.htm http://grammar.about.com/od/c/g/comcomplexterm.htm