Two or more complete ideas that are joined without punctuation create a run-on sentence (also called a fused sentence). The sentence doesn't have to be long to be a run-on -- although the longer your sentence, the more difficult it is to read.
You can:
Separate independent clauses with a period. Doing so will form two sentences out of one.
Use semicolons to form a compound sentence. Think of a semi-colon as an extended breather -- longer than a comma.
Insert a coordinating conjunction, such as "and" or "but" between the independent clauses, which will also form a compound sentence (Rosa Parks was a brave person, but her valor took conviction to stir a nation).
Insert a subordinating conjunction, which will create a subordinate clause in your sentence and create a compound sentence (Although Rosa Parks was a brave person, her valor took conviction to stir a nation).
Examples:
Run-on -- The CDs both of which belonged to the developers were in the test lab.
Correct -- The CDs, both of which belonged to the developers, were in the test lab.
Run-on -- The CDs, both of which belonged to the developers, were in the test lab, and because they were the only available CDs for the new release, the developers were anxious about keeping them clean.
Correct -- The CDs, both of which belonged to the developers, were in the test lab. Because they were the only available CDs for the new release, the developers were anxious about keeping them clean (Actually, the previous run-on sentence was technically correct because of the punctuation; however, it could have been broken to help readability).
