Microeconomics is the study of the economic choices made by individual economic units such as consumers, households and firms etc.
Economists use it to make their arguments more precise and logical.
Econometrics is a branch of economics that deals with the statistical techniques developed by economists to analyze various economic phenomena. For instance, there are techniques to find the exact slope of demand and supply curves etc. A Variable is something whose magnitude can change i.e something that can take on different values. Some of the variables used in economics are prices, profit, revenues, cost consumption, saving and income etc. A Constant is a magnitude that does not change.