A formula simply takes input values, mathematically manipulates them, and then returns a result. All the following are, for example, formulas:
2+2
99-1
5*5000+2*5000
Formulas may not seem like something you should get excited about, but they actually represent the foundation of Excel's power. Why? Because Excel lets you enter formulas into worksheet cells, and then it instantaneously calculates and displays the formula result.