Grills were first worn by hiphop artists in the early 80s, but became widely popular in the mid-2000s when Dirty South rap rose to popularity. They are fitted to the wearer's teeth and can be worn temporarily.
It is not advisable though to let them stay in one's mouth during sleep or when eating because they can be swallowed along with the food accidentally or they may choke someone during sleep.
Grills are most often worn by 18- to 35-year-old urban male hip-hop listeners. However, grills are worn by both men and women of all races, at least to some extent, as well as by celebrities far from hip-hop culture like Marilyn Manson or Travis Barker of Blink-182.