You might have heard this expression before and wonder what this is all about.
In a nutshell, a grammar nazi is a person who knows the difference between "there," "their" and "they're."
More seriously, though, it is a person who uses proper grammar at all times, esp. online: in emails, chat rooms, instant messages and web-board posts; It is someone who believes it's their duty to attempt to correct any grammar and/or spelling mistakes they spot.
If these definitions don't convince you, let's watch this short clip.
(You can find the script below)
Written by Agnieszka Kansy