What is Foucault's theory of panopticism?

Foucault used panopticism to explain how we monitor our behaviour because we've internalised the feeling of being watched. To better understand it, it may be useful to look at where he got this analogy from. The panopticon is a form of prison popularised by Jeremy Bentham, in which prison cells are arranged in a circle with a big watchtower and beaming light in the centre. Through this design, the guard can see all the prisoners, but the prisoners cannot see the guard. Because the prisoners don't know when they're being watched, they'll start to act on their best behaviour at all times. Foucault took this idea, and extended it to everyday life. Let's take shopping for example. CCTV has become so common that we don't even check to see if there's one every time we go into a shop, we just monitor ourselves and don't shoplift. Another very relevant example is Facebook. I'm sure you've heard all the privacy concerns and scaremongering over who can see our messages and statuses, and you may have been more careful about what you post and share as a result. Foucault would say that this is panopticism at work, because we've internalised the threat of being observed, and therefore monitor our own behaviour without the need of an external "guard". There are lots of other examples of panopticism happening today; can you think of how speeding cameras could be explained in a panoptic way?