V-Sync means vertical synchronisation and is used to draw frames at the same rate as your monitor builds up the screen. In general this is the best way to draw motion on a screen without tearing the image. A Cathode Ray Tube (glass tube monitor) builds up a picture with a single dot going blazingly fast from left to right, drawing line after line until it reaches bottom right and starts all over again at top-left. Just explained how a single frame is drawn. Usually this will happen at least 60 times in one second. To the naked eye this is 'almost' impossible to notice. At 60Hz a noticable flickering will make you tired very fast though since your brain has to deal with that phenomenon. That's why we like to use higher refreshrates like 75/85/100 Hz etc.
When not synching frames, it occasionally happens a frame is put out a bit earlier/later than your screen starts building up the image resulting in chopped pictures. This is hardly noticable to someone not familiar with these basics but this tearing of frames is very noticable when seeing horizontal motions. The top half of the screen could be seen out of alignment compared to the bottom half of the screen. The faster the horizontal movement of ingame graphics, the greater the difference between top/bottom parts of the picture and the more noticable the tearing gets. It also won't tear exacly in the middle of the screen, but will roll up or down depending on the difference the framerate has to the refreshrate.
To make my statement more clear, V-sync ON gives a better result to the eye by drawing frames the way they are supposed to be drawn. Synced frames also seem more smooth then unsynced frames, dispite the fact less frames are drawn. Squeezing 200 frames per second into 60 frames seems wrong. So why bother just 60 frames are drawn if your screen does not write more frames in the first place? Maybe to someone things are starting to make more sense now.
For benchmarking etc. you don't want to use any syncronisation since you don't want your monitor to interfere with the outcome of a score.
But after all this explaining, there are some tiny disatvantages of v-sync after all. It takes a little bit of time to syncronize, this is done by pre-rendering some frames before they are actually drawn. A hardly noticable lag occurs by this and the amount of frames being drawn ahead is configurable. Common options are double-buffer / tripple-buffer.
Also, what to do when the framerate in-game drops below 60fps when having a 60Hz refreshrate? The only solution is to drop to 30fps instantly when using double buffer since it only takes measures of two. Tripple buffer allows for synched frames at all sorts of framerates but gives more lag.
When a game runs at 200+fps constantly, the best thing to have would be something like 100hz refreshrate with a double buffered v-Sync. Very close to the most smoothest framerate you will ever experience in a game, a real treat to the eye. If you haven't experienced this once you really missed something
It's so much better then having no v-sync at all. I used to play games like Quake2 with Voodoo2's at such settings, it's almost surreal how smooth things went. IMHO, the time of the smoothest gameplay ever, nowadays a lot of new games suffer from absurd loads/peaks/stalls becouse of inefficient use of recourses and bad programming. Anyway, things always seem better in the past
