There are many types of punctuation. Some of the popular types are commas, full stops, exclamation marks, question marks, semi-colons, dashes and quotation marks (inverted commas). Sometimes, by changing just your punctuation, you can change the meaning of your sentence completely. Look at the next sentence.
The boy said the girl is a thief.
What do you think this sentence means? Who is the thief here?
Well, look at the next two sentences. The words are the same, but the meanings are very different!
'The boy', said the girl, 'is a thief'. The boy said, 'The girl is a thief.'
Notice how the commas break up the sentences in different places. Also notice that when you move the quotation marks, this changes what the speaker says. Let's look at these sentences again.
'The boy', said the girl, 'is a thief'. (The girl thinks the boy is a thief.) The boy said, 'The girl is a thief.' (The boy thinks the girl is a thief.)
The commas tell the reader where the natural breaks are in your sentence. The quotation marks tell the reader who said what. Don't forget to put in the right punctuation, and in the right places, or your reader may read your sentence the wrong way!