A web proxy is a server that acts as an intermediary between a client and a target server. When a client makes a request to a target server, the request is first sent to the proxy server, which then forwards the request to the target server. The response from the target server is then sent back to the proxy server, which in turn sends it back to the client. The main purpose of a web proxy is to act as a barrier between a client and the internet, allowing for tasks such as caching, filtering, and anonymity.