A comma is a punctuation mark that separates items in a list, indicate a natural pause and separates clauses. It looks like ",".
Rules for Commas 
Edit
You use a comma when continuing a sentence after the words (and, but, yet, so, therefore, etc.), and continuing the sentence in a different tone (or comma + conjunction) such as (Ben lost a tooth yesterday, and got an ice cream from Mom because he lost it.) You can only use commas in the case above with two or more sentences.
Related Edit
Oxford comma (last comma in a list)
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