Formula is a statement written with group of mathematical symbols that express a relationship.
Equation is type of mathematical formula with sign equal that separates the left side and the right side of the formula.
The value you put into a formula or equation is referred as a variable. The variable is an item ("thing") which can be measured quantitatively and represented by a number. Variables are called variables because they vary, i.e. they can have a variety of values. If a variable has only one value, it is called constant.