How to display custom content using WordPress Themes

Hello.

Is there a way to display custom content using WordPress Themes design? I am developing site and I would like to use WordPress as a primary content publishing system. I will just add some custom features/catalogs and I would like them to look like the rest of the site. Using WordPress authentication and user management system is not a problem with get_currentuserinfo and such.

Can you please point me to the right direction? I am a PHP developer, but previously focused on pure PHP and VBulletin plugins developing. I have no much experience with WordPress development.