Use "py:match" inside the template you want to include into other pages (eg. a sitelayout.html).
Then you can use "xi:include" in the pages you want the files included in. (eg. best_picture_of_my_cat_today.html)
Now, it's not a "simple include this template here" kind of thing. The py:match can find various parts of the file, and then replace them how they like. For example, a page layout template will match the header and footer with "py:match".
It is best explained with examples, and in the documentation:
Hopefully this explains the genshi way of including a template into a template.