A proxy is used to spoof ones ip address by redirecting requests through it. A person with the IP address of 111 will use a proxy changing his ip address to 222. You will never know where the original request came from, only that it came from the proxy.
Some of the more advanced proxies have rotating ip's, meaning one page load might show the user from the USA, and the next page load may show them from China, depending on the random IP pull from the proxy. IP Banning is really never a safe bet, because as stated above, the ones who you actually need to worry about will not be able to be blocked by there IP address due to proxies / spoofing.