The number of frames (or images) you camera can take per second is expressed as FPS or Frames Per Second.
The current standard for clear, smooth video is 30 FPS, which is the standard frame rate for TV.
Higher frame rate will produce smoother video feeds, lower frame rates can result in choppy or broken movement, but requires less computer power and bandwidth.
Frame rate also is directly related with the size of your video files. A higher frame rate results in more frames, so the video file will be larger.*
When using many cameras, by decreasing the frame rate you will make your surveillance system more responsive. Also videos will be smaller, uploads (by FTP) will be faster and less space will be used in the hard drive.
The optimal frame rate also depends on the kind of scene being monitored. When recording something fast like traffic, higher frame rate would be necessary to smoothly record movement, but in other cases lower frame rate may be acceptable.
* The other factor that affects bandwidth and computer power usage is video resolution. Lower frame rate and resolution will consume less computer resources. A few high resolution cameras at 30 FPS may work well, but if you have many cameras you will need to reduce your frame rate and/or resolution.