A colon (:) separates two independent clauses, but it is only appropriate if the second sentence explains or expounds on the first. A colon can also be placed before a list or a series of items.
An example of a colon separating two independent clauses: “Eventually the truth came out: my sister had stolen my favorite sweater.”
An example of a colon before a list: “On the first day of class, each student is required to bring several items: a pencil, a notebook, an eraser, and a calculator.”