GTK+ and Qt are widget libraries for Linux and other operating systems (including MS Windows). Most of the Linux GUI applications use one of them. They differ in looks (i.e. different open-file dialogues, overall app interface) and functionality (Qt is regarded as a wide-featured).

If you’ve chosen Gnome (or XFCE) as your desktop environment, it’s better to use more GTK+ apps since they itegrate with Gnome better and work faster in this environment (all the libraries are preloaded on Gnome startup). Similarily, KDE users should prefer Qt-based apps for the same reasons.

Most of the Qt apps’ names start with capital “K”. Some examples are KWrite, KLines, KOffice or KWifiManager. There are exceptions though, i.e. Psi, Gwenview or amaroK which are also Qt apps but their authors decided not to follow the masses

Of course, both GTK+ and Qt are Linux apps and you can use them under any DE or WM. It may be however that if they use a “foreign” library, the program may start slower, look a bit different than other apps and not be as integrated as the native ones.