Punctuation clarifies meaning in a sentence. It guides the reader through the material and alerts them to places where the emphasis is. Probably the most misunderstood and misused punctuation marks are the comma, the colon, and the semi-colon.
The colon has a simple purpose: introduction. Most often it’s used to introduce a list of things (Marla bought many kinds of fruit: apples, pears, grapes, bananas, and mangos.) but it also can be used to introduce a single item, adding emphasis (as in the first sentence of this paragraph). However, you should never use a colon after a verb ( Marla bought apples, pears, grapes, bananas, and mangos.)
The semi-colon gets a little more complicated; its usage is often confused with that of the comma. The semi-colon is used when two related sentences are connected in a single sentence without using a conjunction (and, or, nor, so, but, yet). The semi-colon is also used to add clarity when a sentence lists multiple complex items that have commas. It tells the reader what goes together. (Marla bought a big bag of fruit, including apples, bananas, and kiwi; a basket of vegetables, including beans, carrots, and corn; a loaf of bread; and a box of cereal.)
The comma is the most complicated of all because it has so many uses. It is used to separate items in a series, to separate two complete sentences with the use of a conjunction, to separate a nonessential phrase in the middle of a sentence, and to attach words to the beginning or end of a sentence. Using a comma after the item that is just before the conjunction in a list is a matter of style. Sometimes it is necessary for accuracy, (Marla bought different kinds of sandwiches for the picnic; turkey, ham, tuna, peanut butter and strawberry jam and cream cheese. Did she buy a peanut butter and strawberry jam sandwich or a strawberry jam and cream cheese sandwich?), other times you can get away without it. I prefer to use it every time and many grammar experts recommend this.
A great online reference for punctuation (and many other writing topics) is Grammar Girl at http://grammar.quickanddirtytips.com/