That's screen tearing. V-sync will fix it in game. It happens when your FPS is higher than your screen refresh rate. This seems to happen a lot now-a-days with great cards becoming more affordable while 120hz monitors are still $300+. If you get more than 60fps and your monitor is only 58-60hz then it can only display up to the 60 frames and ends up causing the tearing. In some games it isn't as noticeable, you might only see a bit of choppiness in the framerate. I'm sure there are other reasons for screen tearing, but that seems to be the most common one.
This made me a sad panda when I loaded up Diablo III on my new desktop only to find that the game looked "laggy" at 120ish fps. I turned on V-sync and it ran much smoother.
FFXI: DamienSlvrblade Race: Elvaan Nation: San d'Oria Server: Sylph 2002-2007, Phoenix 2012
PUP WHM PLD MNK BLU BST SAM WAR BLM 99 BRD THF NIN DNC SMN RDM 50+
FFXIV: Damien Scott Race: Duskwight Elezen GC: Limsa Lominsa Server: Ultros
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