I installed Ubuntu 12.04 on my laptop today, and when I was updating it using the Update Manager, I saw it downloading qt and gtk libraries. Does that mean I can use them directly? Or will I need to download them from their respective sites and build them?
There's no need to build anything. Maybe those libraries were pulled because they're dependencies of some other package you installed through Update Manager.
So i wouldn't worry about that, chances are those libraries are already installed on your system.