Summarize the following:
Commas can be used to connect two simple sentences and a coordinating conjunction (but, and, nor, or so, for example), while a semicolon cannot.  Example of correct usage: "I love my cat, but he drives me crazy." Example of incorrect usage: "I love my cat; but he drives me crazy." A comma can never be used to separate two independent clauses (complete sentences). This is called a comma splice and should be avoided at all times.  Example of correct usage: "My kitty is cute; he loves to cuddle." Example of incorrect usage: "My kitty is cute, he loves to cuddle."

Summary:
Do not use a semicolon instead of a comma. Do not use a comma instead of a semi-colon.