Frames Per Second (fps) is the number of frame a DVR can capture in a second. Unfortunately, the term is widely abused. The process of capturing and storing video involves three steps:

Video capture: The process of capturing the analog video and converting it into a digital format (typically YUV). The number of times this can be done per second is the most common usage of fps.

Compression: After the video is converted into a digital format, it is compressed using a codec. This process takes a lot of processing power and thus often is the longest. It also may depend on how much motion is in the video stream.

Storage: After the video is compressed it may be stored to a hard disk, and depending on the number of cameras and the bandwidth of hard drive, the step can slow down the overall fps.

Each of these steps has an impact on the true fps number achievable by a DVR. There are some notable exceptions to this. First, an ip camera usually has a specialized DSP chip to assure the camera's ability to perform the advertized fps rate. Some DVRs also use DSP chips and can achieve high fps rates on multiple channels simultaneously.