When combining two complete sentences with a conjunction ("and," "but," "or," "for," or "yet"), precede the conjunction with a comma. Example: Still, the sun is slowly getting brighter and hotter, and ...

Simple sentences are important for making clear points. Be careful though - if you only ever use simple sentences in your writing you don ... They don’t make complete sense on their own.

This means that there are two or more complete sentences fused into one sentence. Use a colon: a colon should be used to join two sentences only when the second sentence explains the first sentence.