Use a colon to join two sentences when the second summarizes, explains, or illustrates the first. As with a semicolon, do not capitalize the first word of the second clause unless it would normally be capitalized.
Dan has one driving force: he loves motorcycles.
A colon can also follow a complete sentence to introduce a word or phrase that summarizes, explains, or illustrates it.
He has one thought: motorcycles.
You can use a colon to introduce a list as long as the material before the colon forms a complete sentence.
Dan loves them all: dirt bikes, cruisers, sport bikes, scooters, and touring bikes.