Calculator

A calculator is a machine which allows people to do math operations more easily. For example, most calculators will add, subtract, multiply, and divide. Some also do square roots, and more complex calculators can help with calculus and draw function graphs. Calculators are found everywhere. A smartphone or other computer can also act as a calculator. 
Some calculators, like the abacus, will work without electricity. Others, like the electronic calculator, use batteries. Some calculators are simple, and can only add, subtract, multiply and divide, and sometimes take square roots. Scientific calculators can do many other things, such as use exponents (or powers or indices) and trigonometry functions.
Pocket calculator.
A pocket calculator is a small calculator that helps people do arithmetic. It is so small that it can be put in a pocket. Its most common use is for addition, subtraction, multiplication, and division. Many pocket calculators are powered by solar cells. They are commonly seen in schools and businesses and can be used quickly and efficiently for simple mathematical problems. Other names are 'miniature calculator' or 'mini calculator'.
Scientific calculator.
A scientific calculator can do more things. It can often use exponents (or powers or indices), pi, trigonometric ratios, and the order of operations. Scientific calculators can also use bigger numbers. They tend to cost more than pocket calculators. Many are also programmable. The user can make a program directly with the calculator, or transfer one from a computer.