Constant:
          A quantity which can be assuming only one value is called a constant. It is usually denoted by the first letters of alphabets.
For Example: Value of  and value of

Variable:
          A quantity which can vary from one individual or object to and other is called a variable. It is usually denoted by the last letters of alphabets.
For Example: Heights and Weights of students, Income, Temperature, No. of Children in a family etc…

Continuous Variable:
          A variable which can assume each and every value within a given range is called a continuous variable. It can occur in decimals.
For Example: Heights and Weights of students, Speed of a bus, the age of a Shopkeeper, the life time of a T.V etc…

Continuous Data:
          Data which can be described by a continuous variable is called continuous data.
For Example: Weights of 50 students in a class.

Discrete Variable:
          A variable which can assume only some specific values within a given range is called discrete variable. It cannot occur in decimals. It can occur in whole numbers.
For Example: Number of students in a class, number of flowers on the tree, number of houses in a street, number of chairs in a room etc…

Discrete Data:
          Data which can be described by a discrete variable is called discrete data.
For Example: Number of students in a college.

Quantitative Variable:
          A characteristic which varies only in magnitude from on individual to another is called quantitative variable. It can be measurable.      
For Example: Wages, Prices, Heights, Weights etc…

Qualitative Variable:
          A characteristic which varies only in quality from one individual to another is called qualitative variable. It cannot be measured.  
For Example: Beauty, Marital Status, Rich, Poor, Smell etc…