What is the difference between --m and m--?

If the decrement operator has placed before the operand it decrement first and other operations later. Example:

m=5      

x = --m;

In this case first decrement the value of m and then transfer the value of m into x.

And

If the decrement operator has placed after the operand it decrement after the other operations. Example:

m=5      

x = m--;

In this case first transfer the value of m and then decrement the value of m.