In general a hockey player should have their skates sharpened once every 6 hours on indoor ice. For outdoor ice the time should be reduced dramatically. The grind to be used depends on where you are going to skate. In cold climates it is generally better to stick with a 3/8" grind, in warmer a 1/2" would suffice. The grind is the radius of the circle that fits into the blade of the skate. (Forgive my poor ASCII graphics)

          |     _____   |
          |   /            \   |
          | /                \ |    < --- BLADE
          ||         --A---||

"A" is the radius of the circle that fits inside the blade's hollow. That is the measurement that "3/8" or "1/2" refers to. Depending on the firmness/softness of the ice that you play on you should adjust your hollow accordingly. For hard ice you need to be able to cut deeper to avoid losing an edge, a 3/8" grind would be best. For softer ice, 1/2" grind would be better to avoid cutting too deep and reducing your glide, thus requiring more effort to skate.

A lot of goalies prefer to use a 1/2" grind, and then dull them by rubbing their blades on wood or rubber. For them a dull blade makes their "shuffle" easier to execute. And since they rarely need to skate and turn at high-speeds the dull blade is acceptable.

Having this hollow also gives the two edges that many skaters refer to. The inside edge is obviously on the inside, naturally the outside edge is on the outside. Putting weight on either edge gives the skater the neccesary friction to stride, stop and turn. The edges of the hollow become rounded and cause the skater to "lose an edge" at high speeds.

The Skate Sharpening Machine generally has 3 wheels on it. One is usually set to a 3/8" grind, another to 1/2" and the last is a flat grinder. The skate is run along the sharpening stone removing the center of the hollow and reforming the two edges.

Attached to each sharpening stone is an "adjustment arm", a diamond tipped swivel-arm that can be set to different radii. These are is swung down across the grinding stone to change the radius the stone will provide. Having two stones on one machine is a good idea in a public setting since most hockey players preffer either one or the other (I prefer a 3/8") while figure skaters and goalies prefer a 1/2".

The flat grinder runs on a horizontal axis (versus the vertical axis of the two sharpening stones). The grinder removes a large portion of the blade and any nicks that may have developed in the blade. Because it removes so much of the blade it is rarely used. Unless there is a very large nick in the skate blade, generally the Sharpener will just make a few more passes on the sharpening stone to remove any small nicks.