GDP stands for Gross Domestic Product. The GDP, is one of the most common measures on the state of the economy for any nation. GDP is the total market value of all goods and services produced in a country for a given time period. The time period most often used is one year, which is then compared to past years as a way to measure the improvement or decline of a country’s economic situation.
There are two ways to calculate GDP: Income and Expenditure approach. As to why economists use different approaches, who knows. Economists seldom like simplicity.