database schema and extensions and versions

I am amazed by the extensibility of WordPress. I am a software developer so, while I am very happy that others have figured out how to make WordPress so cleverly extendable and customisable, I am nevertheless keen to understand it a little from the perspective of the database structure.

My essential question is whether and how and how often chnages to the database schema are made. For instance, does the database schema change between versions of the core WordPress distribution? Do themes, plugins or other customisations sometimes involve additions or changes to the database schema.

I know from experience how important it is to have a well-designed database structure at the heart of any data-driven application. I know how enhancements to the application sometimes require changes to the schema and I also know how much pain that can cause.

I am working in a corporate situation where a WorPress-driven approach has been decided upon as the basis for the next iteration of one of the company web sites. This decison came partly as the result of my positive assessment, so I have a vested interest in understanding the pitfalls if there are any.

So if anyone can offer some high-level design concepts (concentrating on the database schema) or point me to where they are documented I should be grateful.