Live and historical cryptocurrency quotes are pulled from sites like coinmarketcap.com API's.
Market Capitalization is one way to rank the relative size of a cryptocurrency. It's calculated by multiplying the Price by the Circulating Supply. Market Cap = Price X Circulating Supply.
Circulating Supply is the best approximation of the number of coins that are circulating in the market and in the general public's hands. Total Supply is the total amount of coins in existence right now (minus any coins that have been verifiably burned). Max Supply the best approximation of the maximum amount of coins that will ever exist in the lifetime of the cryptocurrency.
Circulating Supply is a much better metric for determining the market capitalization. Coins that are locked, reserved, or not able to be sold on the public market are coins that can't affect the price and thus should not be allowed to affect the market capitalization as well. The method of using the Circulating Supply is analogous to the method of using public float for determining the market capitalization of companies in traditional investing.