Conventional sentences are separated or joined by the appropriate punctuation and words. When a writer joins complete thoughts with no punctuation or with only a comma (omitting a conjunction such as “but” or “and” after that comma) and without doing so for stylistic purposes, the form is nonstandard and referred to as a “run-on sentence” or a “comma splice.” [EDIT]
Conventional guidelines for sentence formation require that complete thoughts that could stand alone as a single, grammatical sentence must be joined by a limited number of strategies:
Conjunction preceded by a comma: He went to the store, and the owner offered him a job.
Semicolon: He went to the store; the owner offered him a job.
Adverbial conjunction preceded by a semicolon and followed by a comma: He went to the store; however, the store was closed.
However, occasionally writers will cast run-on sentences—complete thoughts joined by only a comma—for effect. As with all language usage, purpose is the central issue, not whether or not a usage is “correct” or “wrong.” Fused sentences (complete sentences run together with only a space between) are nearly never used for effect.
We also do not condemn hospitals for not curing the incurable, we do not condemn mortuaries for not raising the deceased.
On-line help with run-ons and comma splices is available here: