The Market Cap is a measure of a company's size - or specifically its total equity valuation. It is calculated by multiplying the current Share Price by the current number of Shares Outstanding. It is stated in US Dollars.
Market Capitalisation only takes into account the value of the company's shares (equity), it ignores the amount of debt a company may have taken on and therefore isn't the best indicator of the company's size. The Enterprise Value adds the net debt to the Market Cap and is a better indicator of the minimum amount that an acquiring company may have to pay to buy the firm outright.