When do you use a colon and when do you use a semi colon?
For example, I could say: Colons are used for introducing something; semicolons are used to join clauses.
A semi colon is used when two clauses can be separate sentences. A colon is used when you're about to list something.
Most people use colons sometime soon after they eat. Semicolons are what happens when somebody gets colon cancer and has part of the colon removed, hence, 'semi'colon.