This sentence is incorrectly punctuated, it is an example of a comma splice.
In English, two main clauses cannot be separated in writing by a comma only - as in the example above. There must be a conjunction added, or alternatively a semi-colon or full stop could be used. The semi-colon, however, tends not to be used in this way in contemporary writing and, though correct, can give the text a fairly archaic feel. A full stop is therefore often preferable, though if the clauses are very short this can give the text a “jerky” style. So :
INCORRECT: The weather was extremely good, we had a great holiday. Again, the sentence contains two main clauses separated only by a comma, meaning it is an example of a comma splice.
CORRECT : The weather was extremely good, and we had a great holiday. Here the conjunction and is added, and the sentence is grammatically correct. The comma here is optional. It’s more likely to be used if the first clause is very long, while here it could easily be omitted.
CORRECT: The weather was extremely good; we had a great holiday. / The weather was extremely good. We had a great holiday. As above - both these possibilities are grammatically correct, but stylistically, because of the fact that the clauses are so short, the option of joining them with a conjunction would be preferable here.
A common mistake is to assume that words like therefore and however are conjunctions and can join main clauses. They're not (they're adverbs) and can't. Either a conjuction should be added, or two sentences must be used :
INCORRECT: Our first plane was late, therefore we missed the connection with the second.
CORRECT: Our first plane was late. Therefore, we missed the connection with the second.
CORRECT:Our first plane was late, and therefore we missed the connection with the second.
You can test your ability to recognise and avoid comma splices with this activity from the University of Bristol.
Want to know more about punctuation in English?