Let's discuss what a NAP is.
NAP stands for:

N...Non
A...Aggression
P...Pact

Simply put, a NAP is when two players agree to NOT wage war on each other.
In a NAP, it is commonly accepted that:
* No hostile actions will be taken against the other player's lands or forces
* No hostile spells will be cast on, or affect any of the other player's lands or forces
* No passage will be granted to someone wishing to engage the other player
* Should one party wish to dissolve the NAP, they will abide by a pre-determined amount of time prior to beginning their assault.

This last point brings us to the other addition to the term NAP, As such, you may see a "3tick NAP" or "NAP3", "5tick NAP" or "NAP5", "Gamelong NAP" or "GL NAP". These terms indicate the amount of time that will be required as warning should you wish to end the NAP.
Of course, a Gamelong NAP cannot be cancelled.

It should be noted that a NAP can be as simple or as complex as the players involved in making this agreement determine as nessecary to their ongoing relationship.
As you need to get the other player to agree to any conditions you set, being to outlandish will quite often be met with a refusal.
This is generally what keeps Naps Sensible.

Lastly, a Nap is very different from an alliance which We'll cover in the NAP Part II message...