A common replacement for the IV or ii chord, the neapolitan chord is a major triad based on the flatted second scale degree (sometimes called II in phygrian mode). Most often, it is seen in first inversion, but can sometimes pop up in root position. As such, the neapolitan chord is most commonly spelled IV bVI bII:
example in C major, and resolution to V
--------------------
----bo--------------
-------------o------
bo
-------------o------
o
--------------------
C: N6 V
As you can see, the root (Db) and the fifth (Ab) tend to resolve down to chord tones in the V chord. To illustrate this, I left out the fith scale degree of the V chord (the root of the N6 chord will rarely move up by step to the fifth of V. Why? Who the hell knows?); you can also imagine that there is a fifth scale degree, if that makes you sleep better at night.
Also, if we're doing 4 part voicing, we tend to double the third (usually in the bass) of the neapolitan chord instead of the more active-sounding root (bII) and fifth (bVI). | 
