V-Sync (Vertical Synchronization) is an option in many games which aims to reduce screen tearing. Do note that V-Sync is not related to the proprietary Nvidia technology G-Sync, though it operates on a same (much simpler when compared to G-Sync or FreeSync) basic level.
Whenever your system (and more specifically; your GPU) is pushing more frames than what your monitor can output it can cause screen tearing (see the image above), which happens because frames get ‘mixed up’ because the GPU is outputting more than what the display can handle. Screen tearing is less of an issue on higher refesh rate monitors, but since most ‘casual use’ monitors have a refresh rate of 60Hz this can be an issue for gamers.
How V-Sync Works
While screen tearing doesn’t necessarily hamper performance it can definitely be mildly to severely annoying, so most games offer the option to turn on V-Sync. This will cause the game to limit the maximum amount of frames it’s outputting in order to match the maximum refresh rate of your monitor, thus eliminating the possibility of the game outputting more frames than what your monitor can handle.
V-Sync works pretty well for the most part, but it does bring about some problems on it own. When your system drops frames (due to large amounts of action on the screen, for example) the monitor will have to wait for the graphics card before displaying the new frame, which causes stuttering and can seem to bring the framerate down to a grinding halt.
Due to the way V-Sync works it also causes (mild to severe) input lag in most games. The game will have to ‘wait’ for the monitor to finish a frame until it sends it to the monitor, so in effect V-Sync delays frames from being shown on screen, making it so that whatever you input to the game can be displayed much (in gaming terms) later on the screen. This input lag will vary from game to game, but it can be as high as 16ms (which is a world of delay in competitive gaming terms) so in general it’s a good idea to treat V-Sync with care and/or only use it with singleplayer games where input lag doesn’t necessarily matter.