This is the PAL sticker that goes on CD covers of music.
Now with film there are many labels of ratings for different films. These rating signs tell the audience what is classed as appropriate for different ages. In the United Kingdom the BBFC (British Board of Film Classification) decides on this.

On websites it is different. Not all websites having warning banners or stickers on them or accounts that protect others from being exposed to their content, because the internet is open to everyone, it is sometimes hard to protect others because nearly anyone can post anything on it these days.
If children want to go on the internet then there are things such as parental locks that can help prevent them from going on anything inappropriate.
-Rebecca Hickey