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V-Sync -
The picture you see on your monitor exists as bytes in Video memory. Like a TV, the electron gun of the monitor starts at the upper left corner of the screen, and sweeps across, lighting up rows of pixels from top to bottom on the screen as it reads the bytes in Video-RAM.
If the picture in memory changes while the beam is still in the middle of the screen, the image can appear to "tear", with the upper part being created from the old picture, and the lower part from the new one.
V-Sync, or Vertical-Synchronization, forces the program to wait until the electron beam is finished painting the current picture before copying the next one into Video memory. It doesn't seem like it would have to wait long, but this happens 60 or more times a second so all the waiting time can slow things down somewhat.
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