USB cameras provide an alternative to frame grabber cards located inside the computer. They have advantages and disadvantages compared to traditional frame grabbers. A general discussion is found in ArtCAM USB 2.0 Camera Performance.
The biggest issue for many is frame rate. Frame rate depends on image size (pixel count) in USB cameras, which is different from frame grabbers. The above article discusses this in some detail, but focuses on USB 2.0 cameras.
FTA also offers the FTA Logitech USB 1.1 camera. This camera will provide 15 frames per second at 640 x 480 and 30 frames per second at smaller formats. This is similar to USB 2.0 performance is some modes, but the 2.0 camera has other modes which are faster. The 2.0 camera also offers zoom and pan which the 1.1 cameras do not.
The most important advantage of the 1.1 camera is that it does not require as much computer CPU time, allowing the use of slower computers. It also draws less power, so is a better choice for battery powered applications.
USB 2.0 cameras are installed in a specific USB connector or port. If you unplug the camera, it must be plugged back into the same connector or reinstalled in the new connector. This is different from 1.1 cameras which are not connector specific.