What is the difference between float and double?

Answer 1

float is fewer bytes in size, and is more imprecise than double.

Answer 2

Float variables require, usually 4 bytes of memory to be stored while double variables needs 8 bytes.

In general a double has 15 decimal digits of precision, while float has 7.

This difference is to have more precision in storing data and perform calculations.

The PI number for example is a infinite number, and calculations using PI are never totally accurate.

So, in your simple homework you should use PI as 3.1415.

But a engineer or NASA physicist would use a more accurate "PI form" like 3.14159265358979323.

I recommend you to read this Stackoverflow topic if you want a different explanation:
http://stackoverflow.com/questions/2386772/difference-between-float-and-double