Two related variables are studied by different methods in Statistics. A special method is applied to study the dependent variable among these related variables by taking time as an independent variable.
The data related to values of the variables changing with time are studied in Economics, Sociology and Business Statistics. For example, population of a country, agricultural production, wholesale price index, unemployment statistics, import-export information, annual production of a certain factory, data from share market, bank interest rates etc are presented with respect to time.
These are called time series as they are dependent on time.