CAPITAL: The word CAPITAL has several different meanings. When you type in all-caps, you are typing in capital letters. States and countries have capital cities (Oklahoma City is the capital of Oklahoma). Capital punishment is a death sentence. Capital can also refer to wealth or other types of assets (so you pay a "capital gains" tax, for example). These are all connected to the Latin word caput meaning "head" and, by metaphorical extension, something that is superior in some way: "That's a capital idea!" (i.e. a good idea).
CAPITOL: The word CAPITOL almost always refers to a building used by a legislative body, like the United States Capitol shown below.