Do not join independent clauses by a comma.
--Strunk and White, The Elements of Style
Using a comma to join two independent clauses is a style no-no. And according to the rules laid down here at QWT, using a semicolon to join two independent clauses is also a no-no. Even though grammarians may permit it.
Here's an example of a typical comma splice error:
It's almost midnight, we can't make it to the bar before last call.
Obviously these are two separate sentences, and the comma serves only to turn two perfectly clear sentences into one run-on sentence. The fix is easy. Either replace the comma with a period...
It's almost midnight. We can't make it to the bar before last call.
...or add a conjunction or other appropriate transition word after the comma:
It's almost midnight, so we can't make it to the bar before last call.
Just don't use a semicolon.