Fetching pure WP data

Hello crowd!

I am totally new to using WordPress, not a PHP newb, though.

The challenge:

A friend has asked me to include WordPress as a means of adding/updating certain content into his own project.

What I would like to do now is just fetch pure DB data from some WP interface, ideally just a plain PHP array of object, and work with it however the existening application prefers.

Example:
I just include a couple of WP files and fetch an array with all the data I need, bypassing the entire template functionality WP offers.

What is basically needed, are a couple of blog categories, maybe a bit of archiving, and a means of displaying individual entries on a single page.

Are there more appropriate ways than just getting them myself from the database? I don’t want to miss out on the RSS / archiving etc. features.

Thx for any help!
LEXI