How does ode45 choose the time step?

Hello,

I know that ode45 uses a variable step Runge-Kutta procedure. I've read that ode45 chooses the time step in order to satisfy that the estimated error at any step should be less than a predetermined amount, specified by the time step. How does ode45 calculate this estimated error? Then how does ode45 use this estimated error to calculate the time step?

Thank you, Kevin