SHARP EDGES (A)
The sharper the groove edges get (there are differences among today's models), the better they grab the ball's cover and produce spin.
LARGE VOLUME (B)
The larger the volume of grooves, the more grass and water they can accommodate being smashed into them. This minimizes the amount of lubricant left to interfere with impact, making it cleaner (and the reason why most club companies only offer box grooves or U-grooves).
NOTE: The USGA has recently made a rule change that can affect your choice to use wedges with box grooves. If you're serious about getting more backspin, you must clearly understand how this new rule may/may not affect you (see sidebar here).