Jitter is what is used to describe a disturbance in the space or timing between transmissions on a network.  This interruption or disturbance can be caused by a number of different factors such as network congestion, improper queuing, or configuration errors.

These errors or congestion will cause the space between transmissions to become lumpy, or they can even cause a delay between the packets and can make the signal vary instead of remaining constant.

You can see this quite prevalently by simply going to YouTube.com.  Just for demonstration purposes go onto YouTube and click on any video.  Do you see the little circle?  That is Jitter.  This is what is commonly referred to a buffering.

The time it takes to buffer the video for you to completely view the video will be determined by the size of the packets and the severity of the disruption in the signal.

If the jitter is large enough it can be out of the range of this buffer and these out of range packets will be discarded.  This will cause some drop outs in the audio part of the video that you are trying to watch.

The main cause for Jitter is congestion in the IP network.  For example, YouTube has millions of users’ online watching videos simultaneously. This will inevitably cause some delays and even failures if their servers can’t keep up with the load of data transmissions.