There are three kinds of pencil sharpeners;

1. Hand Held Pencil Sharpener
   These normally have a small blade mounted in a cone shape metal or plastic receptacle. They are quick and easy to use, the metal versions will last the longest.
    
2. Mechanical Sharpener
   Some are electrical, some are bench mounted and have a crank. In a classroom mechanical pencil sharpeners are invaluable, they are rapid, and often with multiple blades give a fine sharp point as well as automatically adjusting to different size pencils. 
    
3. Knife Pencil Sharpener
   Using a knife such as a Stanley Knife to sharpen a pencil used for sketching is the preferred pencil sharpening method for artists, it gives you greater control over any required desired pencil point, giving you control over the width, shape, length and sharpness of the pencil point