It's called a run-on comma splice.
a comma splice. This occurs when two independent clauses are incorrectly connected by a comma without a coordinating conjunction. This mistake can be corrected by using a semicolon, period, or conjunction to properly separate the clauses.
run-on sentence
contains two or more simple sentences joined by a comma and a coordinating conjunction or by a semicolon> and, but, nor, or for.
The coordinating conjunctions that can be used to combine two complete sentences with a comma are "for, and, nor, because, yet, so."
If it is possible to replace a comma in a given sentence with a period, and wind up with two sentences that are each complete and that make sense as sentences, then the comma that you replaced was being used to splice together two sentences. This is generally a bad idea.
A compound sentence consists of two independent clauses joined by a comma and a coordinating conjunction.
A comma splice is when a two complete sentences are separated by a comma, without a conjunction. This makes it gramatically incorrect. A run-on sentence continues on and on with no clear predicate.
In formal writing a comma is not to be used two join two independent clauses. If comma is used between two sentences consisting of independent clauses it is known as comma splice. Two sentences formed by independent clauses should always be joined by a co-ordinate conjunction or a short punctuation mark such as semicolon if the sentences are closely related.
A compound sentence often consists of two independent clauses joined by a comma and a coordinating conjunction.
If a sentence consists of two independent clauses with a comma between them, it is a comma splice. That is, the part before the comma can stand on its own as a sentence, and the part after the comma can also stand on its own as a sentence, then it is a comma splice. If there is no punctuation there instead of a comma, it is known as a run-on sentence.
He won, he had the best score. This is an incorrect or comma splice sentence -- two distinct ideas joined incorrectly by a comma. Either rewrite as two sentences, or change the comma to a semicolon.
When two sentences are written as one and seperated by a comma, they are called a run-on or fused sentence.