++p increments p by 1 unit and returns the result. This is known as pre-increment.

int p = 0;

int q = ++p; // q=1, p=1.

This is effectively the same as saying:

int p = 0;

p = p + 1; // p= 1.

int q = p; // q = 1.

p++ also increments p by 1 unit, but returns the previous value of p, not the current value. This is known as post-increment.

int p = 0;

int q = p++; // q=0, p=1.

This is the same as saying:

int p = 0;

int q = p; // q = 0.

p = p + 1; // p = 1.

Of the two forms, ++p is marginally quicker because p++ employs a temporary variable for the return value, whereas ++p does not. As such, ++p is the preferred form for looping purposes:

for( int p=0; p<100; ++p);

Which version you use depends upon which value you want returned from the operator, whether it is the previous value or the new value.