Filter: A filter is a pattern used by WebMask to decide which images and web pages not to load in. These will usually be set to match adverts. The pattern is matched against the URL of the image.
Internet address: This is how computers locate each other on the internet. People refer to these via the Domain Name, such as www.oxfam.org.uk. Computers use a thing called the Domain Name Server to convert this into a number (or IP address), such as 126.96.36.199 .
IP address: See Internet address.
Port: Each internet address has multiple ports which it can connect to the internet through. These are numbered and there are standard numbers to use for web access (80), for e-mail (25), for news (119) etc. By default WebMask uses port 8080 for the browser to connect to it - ports can be used for two programs on the same machine to talk to each other.
Proxy: A proxy is something that you instruct to get web pages on your behalf. In the case of a cache proxy, this speeds up web downloads. In the case of using WebMask as a (local) proxy, this removes adverts from the pages.
RFC: Effectively the standards documents for the internet. To find out technical details about many of the terms in this glossary, look up the following RFCs. You will find them at http://www.rfc-editor.org/.