Difference between float and double?

I have assignment to build quadratic equation roots calculator, and i was wondering what was the difference between float and double.

For example

int a, b , c = 1;
float x1, x2, d;

d = (float)b*b - 4*a*c;

Or:

int a, b , c = 1;
double x1, x2, d;
d = b*b - 4*a*c;


What is the difference between them?

 

u wnt be able to judge the difference as long as u get fraction calculations of lower precision..we can say double as a long float.