An NAP or Non Aggression Pact is an agreement between alliances or between an alliance and a player that states that neither will attack the other. These are enforced only by the alliances and/or players concerned, and are often broken for many reasons.
It is a widely held view in the ogame community that only NABS do Naps as a NAB would have difficulty protecting him or herself.
Many alliances have rules preventing its members from attacking and/or spying on each other. In a way, these can be considered as NAPs, but typically aren't called NAPs. Many people consider that no fighting within an alliance is a good idea and it is discouraged to attack a fellow alliance member.