V-Sync (or Vertical Sync) is a graphics technology that synchronizes the frame rate of a game and the refresh rate of a monitor. 'Frame rate' is the term used to indicate the number of times an image is projected on the screen every second (generally, measured in frames per second or FPS). 'Refresh rate' is the number of times in a second that a display refreshes its image.
V-Sync was developed to solve a problem known as 'screen tearing', which is a visual artifact in video display where information from two or more frames is shown in a single screen draw. When a video feeds into the monitor faster than it can refresh, the display will get information from multiple frames in a single refresh cycle. This causes the screen to misalign within the refresh cycle, leading to splitting images that don't match up perfectly, thus visually tearing apart the display.
Implementing V-Sync allows the game's frame rate to match the refresh rate, thus preventing screen tearing. It essentially caps the frame rate to not exceed the refresh rate of the monitor. For instance, if a monitor's refresh rate is at 60Hz, the frame rate in a V-Sync enabled game will not exceed 60 FPS.
However, the use of V-Sync is frequently debated amongst gamers and developers. Among its pros, it fulfills its primary function of eliminating screen tears and gives a consistent visual experience by synchronizing frame output to your monitor's refresh rate. In contrast, its cons include input latency and stuttering. Input latency or input lag happens because when V-Sync is turned on, the GPU has to wait for the monitor to be ready before it can provide the image frame, which can result in a perceived input lag. Stuttering can occur when the graphics card cannot output frames as fast as the refresh rate, causing inconsistent frame times and hence, causing a stutter in the game.
Moreover, V-sync can also lead to performance issues if a computer isn’t powerful enough to maintain a consistent frame rate at the level of the monitor’s refresh rate. To overcome V-sync’s shortcomings, technologies like FreeSync from AMD and G-Sync from Nvidia have been developed that provides dynamic frame syncing without the issues presented by traditional V-Sync. Thus, whether or not to use V-Sync would depend on personal preference and a balancing act between the quality of the visual output and the demands it places on system performance.