There are two sub components of economics, microeconomics and macroeconomics. Microeconomics is like taking a microscope to economics, and focusing on one individual or firm. We try to use economics to predict their behavior. This is where utility theory, and production theory come into play. Macroeconomics focuses on the aggregate economy, and looks at countries and how they interact. Macroeconomics looks at GDP (gross domestic product), employment, inflation, and trade.
The core idea of economics deals with scarcity, which is covered in another post: on scarcity.