Those are what are commonly call 'artifacts'. Basically, it is 'noise' being introduced into the video signal. Most often caused by failed or failing video RAM. No real way to correct it short of replacing the card. Note that an underlying cause is often a low quality and/or underpowered power supply.
PS: Video artifacts can also be an indication that your graphics card is being overclocked too aggressively.
PS: Video artifacts can also be an indication that your graphics card is being overclocked too aggressively.