Main tab gives you access to options you need to start working with PhotoMix (create new project, open saved project, etc.) and options you need when your work is finished (save project, print project, save as photo, send via email, etc.).

Photos tab allows you to add photos you'll be working with. Using Photos tab you will always have photos you need at hand.
Learn how to add photos...

Borders tab opens borders library. Use a drop-down menu to find borders from different Addins.
Learn how to apply border to photo...

PhotoMix features unique context toolbars. Whenever you select a photo, or a text layer, or a clipart a context menu will appear allowing you to move, align, and perform other actions.