An open proxy is a proxy server which is accessible by any Internet user.
Generally, a proxy server allows users within a network group to store and forward internet services such as DNS or web pages so that the bandwidth used by the group is reduced and controlled. With an "open" proxy, however, any user on the Internet is able to use this forwarding service.

By using some open proxies (the so-called "anonymous" open proxies), users can conceal their true IP address from the accessed service, and this is sometimes used to abuse or interrupt that service, potentially violating its terms of service or the law; open proxies are therefore often seen as a problem. However, anonymous open proxies are also used to increase anonymity or security when browsing the web or using other internet services: a user's true IP address can be used to deduce information about that user and to hack into his or her computer. Furthermore, open proxies can be used to circumvent efforts at Internet censorship by governments or organizations. Several web sites exist which provide constantly updated lists of open proxies.[1]
It is possible for a computer to be running an open proxy server without knowledge of the computer's owner. This can be the result of misconfiguration of proxy software running on the computer, or of infection with malware (viruses, trojans or worms) designed for this purpose.
Many open proxies run very slowly, sometimes below 14400 baud (14.4 kbit/s), or even below 300 baud, while other times the speed may change from fast to slow every minute. Some, such as PlanetLab proxies, run faster and were intentionally set up for public use.
Because open proxies are often implicated in abuse, a number of methods have been developed to detect them and to refuse service to them. IRC networks with strict usage policies automatically test client systems for known types of open proxies.[2] Likewise, a mail server may be configured to automatically test mail senders for open proxies, using software such as proxycheck.[3] Increasingly, mail servers are configured out of the box to consult various DNSBL servers in order to block spam; some of those DNSBLs also list open proxies.

A closed proxy is one that is only accessible to specific individuals. It is possible to use someone else's computer in order to hide one's identity and/or location.