There are four main types of data: time series data which records values over time, cross-sectional data which records values at a single point in time, pooled data which combines time series and cross-sectional data, and panel data which is a type of pooled data that tracks the same observational units over time. Panel data allows analysis of both differences between observational units as well as changes within units over time, making it a combination of cross-sectional and time series data. It is often used to study things like changes in households or businesses using a unique identifier to follow each unit longitudinally.