Here's an example of a quantum system — me. My mass is about 80 kg and I walk about 3 m.p.h. or about 1.3 m s^-1 on average, which gives me the miniscule deBroglie wavelength of about 6.2 x 10^-36 m and explains why I don't diffract too much when I walk through a doorway. This is how people thought about quantum effects pertaining to moderate sized objects like me in the middle of the 20^th century. But physicists are finding that this is the wrong way to think about large collections of atoms. Even large collections of atoms can exhibit quantum behavior, as several recent experiments have shown. Roger Penrose even believes that our minds rely on unknown quantum processes that occur within our brains. So it seems that quantum phenomena may play an important role even for you and me and other items on our scale. And it certainly plays an important role on very small scales and in the vicinity of some very large scale objects such as black holes.