Censured?

I've been always curious about how and why movies are censured but never really found out because I've never ever looked at the rating of the movie or game that I'm gonna watch/play. I was raised in a catholic family but  my parents never prohibited me to watch rated movies. Obviously when it comes to pornography it was different.. I guess cultures are different? When I moved to Florida about 3 years ago I met friends in high school that every time we would go out to the movies they would worry about the movie's classification. I just didn't care, I just wanted to watch that movie everyone was talking about. In Peru I have never been in a situation like that with my friends. I don't think I'm the only one who affirms that similar scenes as the ones in rated movies or the ones prohibited to under 18 people, are shown in TV at any time. Also, there are movies with similar content that are not rated. So what is the point? Here is the definition of rated...

R- under 17 with the company of their parents. Adult content. Inappropriate language. Intense violence, nudity, drug abuse.

In my opinion parents should allow their children to watch any movie they want, but not to the extremely point of letting at 6 year old kid to watch movies with nudity or violence scenes. Young people want to be able to watch horror, action or even comedy movies, we love that. Oh! but wait we can't because they are rated! IT IS JUST A MOVIE !