If two independent clauses are to be joined into one sentence, they should be separated by a conjunction or a semi-colon, or possibly even a conjunction and a comma. They can also be separated into two sentences by a period. Using a comma causes a comma splice.
Koala bears are not actually bears, they are marsupials.
I am not angry with you, I am not happy with you, either.
I’m thinking of skipping English class, it’s really boring.
Exceptions: Comma splices can be used for artistic or poetic effect, as when one is connecting several short independent clauses. Don’t do this in a formal composition, though; it’s only for creative writing. (If you’re going to pull this off in formal writing, try using a semi-colon.)
She was beautiful, she was gorgeous, she was ravishing.
Comma splices may also be used if the two independent clauses are somehow contrasting, as when following a statement with a question.
You are coming to the party, aren’t you?