A person on the Internet that uses proper spelling and grammar. They usually dwell on forums or other social websites and correct the grammar/spelling of, or insult those who have improper grammar or spelling.
The Grammar Nazi will be hated by the person targeted by "He who spells". The "victim" will usually resort to useless insults.
It is a sophisticated form of trolling. The end result is serving the duty of the Grammar Nazi, and causing the idiot to be enraged.
(Assuming this takes place on YouTube. GN = Grammar Nazi.)
Person: "dat vid was sooo tight man u r a genius. da vid iz funny cuz its tru"
GN: "Excuse me. Did you take elementary school English classes? It seems that your sentence has many spelling and punctuation mistakes.
Person: "*response or private message* hey man wat da hell iz wrong with u? just cuz u no how 2 spell doesnt make u ne smarter. u r probably a virgin or gay. hahaha. u cant even get gurlz. deres no reason 2 b a grammer nazi"
GN: "Well, it might not make me smarter, but it definitely makes you dumber. Don't even get me started on the mistakes in that response."