Common Core Standards: Math
High School: Functions
Linear, Quadratic, and Exponential Models F-LE.1c
c. Recognize situations in which a quantity grows or decays by a constant percent rate per unit interval relative to another.
Students should also know when to apply exponential functions and when to apply linear functions. As we can see, linear functions are all about constant growth. The y values change a constant amount for every x value, and that's it. End of story.
Exponential functions, on the other hand, love extremes. They can get crazy-big or crazy-small, but the key is that it happens crazy-fast. Their growth is always increasing (or decreasing, when the base is less than 1). Exponential functions are useful in describing things like bacterial growth and radioactive decay.