Jitter (CRT issue):

Interference due to the presence of alternating electromagnetic fields, such as from ac power lines, can deflect the electron beam tracing CRT image, creating vertical and horizontal displacements which vary over time. This is known as "jitter".

Depending on the jitter frequency, the human eye will perceive it as motion (~30 cycles per second), and blurring (>30 cycles per second).

Jitter is very difficult to correct, because often-times the source is external to the monitor. Sometimes the use of "jitter-boxes" will help reduce the interference. Sometimes reorienting or repositioning the monitor may help.


Ringing:

Ringing is a signal processing artifact, and it generally occurs along edges of high contrast.

Normally, video processing circuitry suppresses ringing, but due to a variety of reasons, this suppression or "dampening" action becomes ineffective. If your graphic card or monitor is ringing, you will see echos of the lines in the cross-hair.