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.