Exponential functions have the unknown in the exponent (power).

Exponential graphs have an asymptote at $x=0$ and a $y$-intercept of $1$.

$y=a^x, a > 1$



$y=a^x, 0 < a < 1$



Exponential functions where $a<0$ are beyond the scope of A-Level Maths.

The natural base

There is a special case when $a = e$.

$e$ is known as the natural base and is equal to $2.71828182846...$ It is an irrational constant (similar to $\pi$).

$y=e^x$