1. Ceramic and AlNiCo ( alloy containing Aluminum, Nickle, Cobalt)are the metal types used for the magnets. There are several different combinations of aluminum, nickle, and cobalt that are used in pickups and that's when you see Alnivo V or Alnico IV... that's the different types of alnico alloys. The main difference is the tonal characteristics of alnico magnets are generally more balanced and full. Ceramic magnet pickups are generally geared towards high gain pickups... for instance, the dimebucker (Dimebag Darrell pickup) uses ceramic magnets. A more classis balanced tone from alnico, more aggressive and distortion friendly tone (but lacking in clean tone character) from ceramic.

2. Shielding does make a difference but not so much that it would be immediately audible. But yes, open and closed basically means whether or not they have the metal covers on them.

3. "Hot" is a designation given to pickups that are overwound... Within the design of a pickup there is wire that is wrapped continuously around a magnet. The amount of times the wire is wrapped generally dictates how "hot" the pickup is. This will carry a higher output signal and cause the pickup to have some natural distortion to it... A good example of "hot" pickups are the Texas Special pickups that are used in the SRV Stratocasters. They're basically a standard Alnico strat pickup that has been overwound, giving them a slightly hotter sound. Makes them play really nice with overdrive.