capital and Capitol - the difference
 
Capitol is the building where the US Congress meets. 
 

    Grammar Checker

Capitol

Many people (particularly non-Americans) who see the word Capitol in writing assume it is a typo of capital. In fact, The Capitol is the building that serves as the seat of government for the United States Congress. It is located in Washington on top of Capitol Hill.
 

The United States Capitol

Capital

The word capital has numerous meanings. As a noun, it denotes: (1) a city that is the seat of the government for a country or state (2) an amount of money or property.  As an adjective it denotes (1) an uppercase letter (2) first rate, excellent (3) main, principal.

Examples:

The Capitol is the main building of the US Congress.
(Capitol is also known as the Statehouse.) 

The capital of England is London.

That was a capital speech you made from the Capitol.
(capital = first rate, excellent)

Do you have any capital invested in her business?
(...any money or property invested)

Our capital concern is that everyone gets fed during the electricity failure.
(...main or principal concern)

In German, all nouns start with a capital letter.
(Ich habe eine Katze = I have a cat.)

 
ALWAYS USE A CAPITAL FOR CAPITOL 

The word Capitol is a proper noun (i.e., the name of something). Therefore, it should always be written with a capital C.  
 

See also:

What are nouns?
What are verbs?
Glossary of easily confused words
Glossary of common errors
Glossary of grammatical terms