What It Is
Simply put, the frame rate is the number of pictures a webcam can take and how quickly it can then transfer them to the computer's screen. Frames are measured in "frames per second" (or fps). So if your webcam says it has a 30 fps, that means it can take 30 pictures every second and then transfer them to the computer screen.
How It Works
Once a picture (or frame) is captured by the webcam, it creates a JPEG file of the still image. When the webcam has a low frame rate (15 fps or below), the webcam can only transmit a series of these JPEG still images. When the frame rate is higher than 15 fps, the webcam can actually stream video using the computer's Internet connection.
Frame rates generally range from 10 fps to 60 fps. You should try to stay closer to 30 and higher if you don't want very choppy video.
Note: In order to stream video, you not only need a webcam with a decent frame rate (15 fps is the absolute bare minimum), but you also need a high-speed Internet connection.
Although the number on the box may say one thing, what your webcam actually captures may be different. Certain things can affect a webcam's frame rate, such as the capabilities of webcam's software program, exactly what you are trying to record, the resolution of the webcam, and even the amount of light in the room. Likewise, running multiple devices via your computer's USB ports can also slow down the frame rate.
It's safe to say that frame rates will continue to rise in conjunction with the webcam's resolution, which also determines how sharp the video will be. As high frame rates and high-definition resolutions become more commonplace, the prices will drop and the low-frame-rate webcams will likely disappear. It will not be long before 30 fps becomes the bare-bones minimum for an entry-level webcam.