Using a semi-colon

Semi-colons are used to join two sentences that are closely related.  Sentences can always be ended with a period, but a semi-colon suggests a stronger association between the two ideas.

Unjust laws are really unfortunate; just laws make us happy to be alive.

I sent the letter by courier yesterday; the post office estimated arrival in two days.

I hate singing songs around the campfire; I prefer throwing explosives into the flames.

Using the colon

In addition to being a large part of the body, the colon is used to set up lists or elaborations.  It is often used after a complete sentence to bring attention to a list of items.  It can also be used if the sentence following summarizes the first.

            There were four Beatles: John, Paul, George, and Ringo.

I always like to bring a few items with me in the woods: a meat sandwich, a compass, twelve sticks of TNT, and a large blowgun.