We use scientific notation to represent extremely small numbers or extremely large numbers. For example the graviational constant is $G=6.672\times10^{-11}$This means that G is extremely small. Here's how it would look without the scientific notation. G = 0.00000000006672 So basically you need to use scientific notation to simplify stuff because when dealing with real world problems all the constants, and many actual numeric values are in the domain of such sizes. The parts of an expression are the number 6.672, and the power of ten, that is 10^x.