1. A run-on sentence is one with two or more independent clauses (i.e., complete sentences) joined without an appropriate punctuation or conjunction (Wikipedia). Here are a few examples:
My dog likes to go for walks I walk him almost every day.
The carnival rides were fun I like the ferris wheel the most.
My trip to Washington DC was excellent I was near the Whitehouse I wanted to take the tour.
He wasn't a teacher he was the school janitor.
To avoid run-on sentences, follow these rules:
1. Join the two independent clauses with one of the coordinating conjunctions (and, but, for, or, nor, so, yet), and use a comma before the connecting word.
2. When you do not have a connecting word (or when you use a connecting word other than and, but, for, or nor, so, or yet between the two independent clauses) use a semicolon.