You don't need to understand search.class.php to learn how a search works. Look at one of the plugins, e.g. the staticpages plugin, instead.
A plugin has to implement two functions: One that provides the text for the dropdown on the advanced search page and one that does the actual search. In the staticpages plugin, those are plugin_searchtypes_staticpages and plugin_dopluginsearch_staticpages. The latter looks complex, but it isn't that complicated really - most of the code is to handle the different search types ($keyType).
So I'd say you should try and write a simple Geeklog plugin. There's a nice
Beginner's Guide to Geeklog Programming in the wiki that should get you started.
HTH
bye, Dirk