How to use a semi-colon
Semi-colons are used to connect 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.
Examples:
Unjust laws are really unfortunate; just laws make us happy to be alive.
I sent the letter by courier yesterday; the post office estimated that it would arrive in two days.
I hate singing songs around the campfire; I prefer throwing explosives into the flames.
How to use a colon
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.
Examples:
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.