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Yagsbook is an XML format specifically designed for the mark up of roleplaying game content. It was originally based on DocBook which does much the same for technical documentation. A Yagsbook document can support multiple types of game systems (allowing d20 and GURPS content to be included side by side for example) and rendered to multiple output formats - currently HTML and PDF are supported.
The purpose of Yagsbook is that document content can be structured in such a way that accessing and manipulating it becomes easy. If a set of rules is written in Microsoft Office, then a chapter listing skills is just a sequence of text, possibly with some fancy headings. It is very difficult to do anything useful with that text - for example pull out a list of skills sorted by group, or filtered according to genre. With Yagsbook this becomes possible.