Because it is a separator, you will find commas separating the items in a list. If you leave out the comma, things can get confusing!
- without the comma: I love cooking my family and my pets. (eeeek!)
- corrected: I love cooking, my family and my pets.
The comma is also to separate the different elements of a sentence. Again, if you leave out the comma, the results can be fatal:
- without the comma: Let's eat Grandma. (eeeek!)
- corrected: Let's eat, Grandma.
Unfortunately, it's not always easy to know just when a comma is required. You do not mark every single new element in a sentence, but only some of them. Which elements gets marked off with commas is a matter of style more than grammar. There are some general guidelines which can help you to decide when to use a comma in standard written English, but the comma rules are not as hard-and-fast as some other rules of English writing.
Meanwhile, it's important to note that commas are not strong enough to separate two sentences. To separate one sentence from another you need to use a period or a semicolon; a comma is not enough. If you try to use a comma to separate two sentences, the result is what is called a run-on sentence. More specifically, it is a comma splice. The comma splice is one of the most common errors you will see in English, understandably so. Because commas are separators, it is tempting to use them to mark the separation between two sentences, but that is not a grammatically correct use of the comma.
As proof of the comma's complexity, here is a list of the pages at this site that cover the many different ways in which you can use commas to separate elements inside a sentence:
- Commas and Interjections
- Commas and Vocatives
- Commas as Parentheses
- Commas and Relative Clauses
- Commas and Starter Elements
- Commas and Final Elements (Free Modifiers)
- Coordinating Conjunctions
- Correlative Conjunctions
- Subordinating Conjunctions
- Conjunctive Adverbs
- The Ten Rules of Quoted Speech
- Five Uses of "So" in English
- Comma Splice (Run-On Sentence)