As a start, here's an overview of changes between Genoa and Siena in terms of architecture:
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Core modules (forums etc.) moved from /system to /modules - more about directory structure
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Siena has 'modules' and 'plugins', which together are called 'extensions'. They have the same structure and functionality, but modules are usually stand-alone pages, while plugins are used to enhance existing functionality (core, modules or other plugins).
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Siena uses PDO, which means SQL queries are done using $db->query() instead of sed_sql_query(). - more about the database layer
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Siena supports caching (10x faster pageload), but this is optional. - more about caching
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Siena has install.php, which automates a lot of the installation and upgrade procedure (also for extensions).
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Siena uses index.php as entry point for (almost) all requests. This means modules do not need their own file in root. Existing files are kept for backwards compatibility (e.g. plug.php). Extensions are now called using index.php?e=myext
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Siena has several collections of helper functions which we call APIs. They are the files in /system and include forms, caching, file uploads and extrafields. I especially like the Forms API, which makes extension development a lot easier. - some API documentation here (needs to be improved)
If you want to have a look at the Siena directory structure and files without having to download and extract the package, have a look on Github.
@Kingsley: It's no problem if you're not a pro developer, I once started as a newbie too. As long as you're enthousiastic about it, have no problem spending your time on it and you want to learn this stuff, you'll be ok. Trustmaster and I are here to help you if you have questions.
