Grain in movies

[Skygod]

I see a lot of reviews refer to the grain. I believe this is referring to that moving/grainy look of movies when you look very closely.

I remember the movie 300 had tons of this, and that most movies have some measure of it.

question is: is this intentional, and what is the source of it? Why is the movie 300 heralded as a great visual achievement with so much of that "grain" going on?

Thanks