A constitution is a set of rules and procedures that organizes and structures the functioning of an organization. This means a constitution could be written for a country, a state or province, a business, a club, or any other organization that needs a standardized procedure to carry out its purposes.
Democracy is a type of government. In a democratic government, the people being governed have the power and make the decisions for the government, either directly by voting personally for every decision that is required of the government or through representatives who are elected by the people and given the authority and responsibility to vote on behalf of the people.
In the United States, the Constitution establishes a representative democracy as the form of government for the USA. This can also be called a constitutional democracy since it is designated as the type of government for the United States in the Constitution. Citizens who meet the qualifications to do so may vote in elections for representatives to the Senate and the House of Representatives. These elected officials vote on behalf of the citizens who elected them to make the decisions needed to run the government.