Linear growth is always at the same rate, whereas exponential growth increases in speed over time.
A linear function like f(x)=xf(x)=x has a derivative of f'(x)=1, which means that it has a constant growth rate. No matter how long the object or population is growing, no matter what its size, the growth rate will always be 1 - no exceptions.
On the other hand, an exponential function like g(x)=e^x has a derivative of g'(x)=e^x. This means that as x gets larger, the derivative also increases along with it - meaning that the graph gets steeper and the growth rate gets faster. In fact, the growth rate continues to increase forever.