Re definition of a "belter"...
Though it seems like some people just mean "singing loud" when they say belting, I think what most of us who are obsessed with this subject mean, when we say "belting," is that someone is singing in their "chest voice" as opposed to what non-opera people like me call "head voice." It's a result of how the sound is actually produced by the vocal chords (everybody has both a head and chest voice).
A singer can choose to sing in head voice or chest on most lower notes; as you go higher, it becomes harder and harder to sing in chest (belt) and most singers reach a "break"; above their break they can't belt anymore and can only sing in head voice. Classically trained women singers and particularly opera singers usually sing ONLY in "head voice" or "legit," while a lot of broadway ladies, r&b, and pop singers (like the American Idol ladies, yup) sing mostly in their chest voice. The pet peeve of people who love belting is when a singer wimps out and goes into head voice on a high note that is supposed to be belted. Looots of singers do this... and it is always very disappointing.

Alice Ripley, for example, usually a huge belter (Side Show) did a production of "Song and Dance" in Washington D.C. a year or two ago, in which she sang almost everything in head voice. God knows what possessed her. It was the most disappointing show I think I've ever seen.
Chest voice is a sound a lot more like a regular speaking, or shouting(!) voice, more throaty or nasal; head voice is that clearer, often higher, less earthy tone of opera singers and say, Julie Andrews in the Sound of Music. In Wicked, for example, Kristin Chenoweth, who is classically trained, usually sings in her head voice; Idina on the other hand almost always "belts" her big notes in chest voice. (In part of their duet on the Tony's, though, both of them belt in chest voice!) Or think Les Mis: Eponine and Fantine belt in their big songs; the adult Cosette sings only in head voice.
The confusing part comes when women "belt" higher than maybe a B or C (or really high belters, above a D or E!)... because then the voice placement changes, and the sound really resonates more in the head--so I don't know if it's really "chest voice," but it continues the same belty sound and it's definitely not head voice. Some singers also "mix"; they sing in head voice with sort of a focused, belty sound, or they somehow do a kind of combination and it's hard to tell if it's chest or head voice. Liz Callaway and Ann Runolfsson are experts at mixing.
Re Guys belting, as I understand it the belt/head voice distinction is pretty much identical to a guys regular voice/falsetto distinction. Head voice= falsetto. It's just that guys don't normally develop their falsetto into a full singing voice.
Wow, sorry to write so much.... Hope that helps?