A quantity which retains the same value throughout a mathematical process is called a constant. A variable is a quantity which can have different values in a particular mathematical process.
It is customary to represent constants by the letters a, b, c and variables by x, y, z.
A variable is an independent variable when it has any arbitrary (independent) value. A variable is said to be dependent when its value depends on other variables (independent). For example,
A = πr2 (area of a circle)
For different values of r, you get different values A. Thus, the quantity A depends on the quantity r. Hence, A is dependent variable and r is an independent variable.