well, in Japanese phonology, the ん is called a moraic nasal, and is very interesting character, as it (much like the sokuon) has no individual pronunciation (ie you never see っ or ん by themselves, or at the beginning of a word).
The ん character has a variety of romanizations, because it actually adopts the place of articulation of the following sound (both vowels and consonants), but is realized as a nasal sound in that place of articulation. It's an "n" in cases like さんぜん (sanzen) and みんぞく (minzoku), and an "m" in words like にほんばし (nihombashi). There are several other phonetic realizations of this character; it's called assimilation.
I've gone into more detail on this subject in
this thread. Here's a
direct link to my post regarding ん.