[texhax] The effect of the space character

[P. R. Stanley]

Hi folks,
The following extracts are from chapter 14 of the TeXbook.


"... you shouldn't leave any blanks next to the tilde, since they 
will count as additional spaces."

What kind of blanks is the author refering to here? I thought 
additional space chars (ASCII 32) had no impact on the output. Could 
he be refering to macros such as the quad, qquad etc.?

"If you put tilde at the very end of a line in your input file, 
you'll get a wider space than you want, because the  return  that 
follows the tilde produces an extra space."

Is this extra space horizontal or vertical?  If horizontal, how would 
a space differ visually from the rest of the page with no printing?

Many thanks
Paul