A proxy is something that allows many people on the internet to view websites like Facebook and MySpace if they've been banned. Now typically you shouldn't be getting yourself in trouble and banned at these websites but it happens sometimes, or you may also just like having the privacy that you get from using a proxy. Nobody will be able to track your IP address when you're using a proxy because you're not actually connecting to the server of the website you're just viewing the pages on the website.
There are thousands if not millions of people who need to use proxies due to being banned from websites but the number one reason I believe proxies are used is because many schools block websites but with a proxy you'd be able to access anything. Schools hate proxies but it's impossible to block or ban every proxy which is why they are so popular. Many proxies get banned from lists or they shutdown and then a new proxy comes along that is just as good and can offer the same service.
To find out which proxy servers you can use to view websites you should consult the help of a proxy list which lists numerous of the top proxy websites online. These lists typically need to update very often because proxies are changing all the time, it's hard to keep up with what proxies are working and what ones aren't, but then with a proxy list you can find the updated proxies to use at all times whenever you need to know.
A proxy list is very valuable to people because without them you wouldn't know which proxies were open and which were shut down. Proxies change weekly and some shut overnight due to unexplainable reasons which is why proxy lists need to be updated so often and why they are so important to us proxy surfers.