Open proxy may have different interpretations, e.g., some geeks deem web-based proxy as open proxy; others may confuse open proxy with unauthentic proxies.
Open proxy usually is similar to web-based proxy (e.g., using http protocol), but there are some differences. Web-based proxy retrieves resources for the client, and at the same time some modifications were also made. For example, most web-based proxy scripts rewrite the java script and act as client directly. But most open proxy only fetch the resources and do not make any modifications.
Because open proxy uses http protocol for the connections between the client and the proxy server. This makes it different from tunneling, or socks proxy.