Information is posted on a regular basis on rec.games.hack about where to get the latest version of Nethack. If you have access to Compuserve, it is stored in the Gamers' Forum there. Describing exactly what alignment is and what it entails is tricky, however. Expressed simply, alignment is what side you've aligned yourself with - good, evil, or neither. A chaotic character will probably kill anything that walks, just for fun, and will tend to rob shops as a principle.
Neutrality tends to be a refusal to take sides.
A character who is neutral probably won't kill creatures that aren't bothering it, but won't have any qualms about killing those that do. A lawful character would never even think of robbing a shop. This number is increased for every action you perform which is in keeping with your alignment, and decreased for every action you perform which would be against your alignment. The chart below shows what actions can affect your alignment and by how much.