From Wikipedia:
The Nvidia Quadro series of AGP, PCI, and PCI Express graphics cards comes from the NVIDIA Corporation. The cards' designers aimed to accelerate CAD (Computer-Aided Design) and DCC (digital content creation), and the cards are usually featured in workstations (compared to the NVIDIA GeForce product line, which specifically targets computer gaming).
Just like merandos stated earlier, these cards are designed for different purposes.
Quadro 600 will be better for rendering and stuff like that, whereas the GTX 690 will be superior when it comes to gaming.