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 players lands or forces
* No hostile spells will be cast on, or affect any of the other players 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 the 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 next

A GL NAP has simply declared that they will not wage war with the other party for the entirety of the Campaign although NAPs can sometimes evolve into Alliances, but they are two very different Diplomatic Entities.

An alliance may be between 2 players or even several players within a game, it may have goals like trying to get all players within the alliance into the purse for instance.

If you do find yourself needing to cancel a Nap to assist an Ally or for any other reason for that matter, here is how the process works.

In the case of a 3 tick Nap, Termination must include a three cycle warning period before attack orders are given.
As an example, if warning is provided on cycle 15, movement orders may be issued on cycle 18.
As seen above, the orders may be issued on the third cycle of warning, however, the battles will not be able to be viewed until four cycles after notice of cancellation is given.
For the most-used definition of a NAP, please follow this link.
http://www.madnergal.com/en/oldelrads/Scrolls/NAP3.html If you've any questions on NAPs or Alliances, please do not hesitate to contact your coach or ask Questions in the Newby Q&A NG or even Chat, even most lurkers will answer a newbs queries :)

while there are several good resources for pre written NAP agreements that you can check out, look at it this way.

A NAP is an agreement between to players, usually brought about by a discussion between the parties as to what each of them want from said peace agreement, this may include or exclude many things & every NAP has the flexibility to be whatever you want it to cover, of course you have to get the other guy to agree & this is generally what keeps NAP agreements sensible.

Some players will want to be very detailed while other will be very casual, it may be as simple as "want a NAP?"

So what is a basic NAP then?

A basic NAP would include things like:
1. you don't attack each other ;) 2. no casting spells that effect your NAP partners forces or lands without asking 1st. 3. you do not allow anyone to access your NAP partner through your regions.

Other things you should consider.
Duration, how long is it, 3, 5 or GL, you may want to include fury farm or temple swap agreements, mutual enemy agreements (seen more in team games than Indy games) you might want to include an inactivity clause? ie: If you go inactive for 2 ticks, the nap will become void & must be renegotiated should you return.

Also be aware that while most players will honour their word, should you intentionally or mistakenly attack or effect a NAP partner, they may decide that you have breached your NAP & may further declare war upon you.

Allow honest mistakes.