Why is 120 Hz better?
In order for a 120 Hz monitor to display 120 or 144 frames per second, you need to use a dual-link DVI cable, because only those have enough bandwidth. It's necessary since twice the amount of data needs to be transmitted.
In order for a game to benefit, it needs to be able to achieve a frame rate of at least 120 frames per second. This can be a challenge even for powerful graphics cards in the more recent and demanding games if the settings aren't very low.
Our frametime tests can help here. 120 fps means that every 8.3 ms a new frame is compiled on the screen. If each frame in a game takes no more than 8.3 m with a particular graphics card, it means that that card is fast enough to run on a 120 Hz monitor. The Nvidia GeForce GTX Titan is the fastest single-GPU card that we've tested, and even that one isn't able to achieve 120 fps in Full HD consistently in the latest games. So if you want to get the most out of a 120 Hz monitor and enjoy the most demanding games, you will have to buy multiple video cards, turn down the quality settings, or accept lower frame rates.
That doesn't mean a 120 Hz monitor is useless if you're graphics card isn't fast enough to consistently produce at least 120 frames per second. There are other advantages, thanks to Vsync.
The speed in which a graphics card produces frames isn't automatically in sync with the refresh rate of a monitor that's either 60 Hz or 120 Hz. The GPU can be slower or faster, and the speed isn't consistent depending on how much is happening on the screen.
Most games have a feature called Vsync (Vertical Sync), which lines up each new frame with the monitor's refresh rate. This prevents tearing, when partial frames are displayed. Vsync therefore benefits the quality, but there is a downside, which is why many gamers disable this feature. It can potentially slow down a game, on 60 Hz monitors the delay can be up to 1/60 second, or 16.7 milliseconds. Avid FPS gamers claim they can notice this (which is often confused with input lag, which is something else). Another drawback is that if your graphics card is unable to consistently produce at least 60 frames per seconds, you can experience stuttering.
Without Vsync, frames are displayed as soon as they are ready. This can result in parts of two or more frames appearing simultaneously. If your graphics card is much faster than 60 fps, it can even happen that you see parts of multiple frames. During the 16.7 millisecond refresh cycle, when the frame is compiled line by line from the bottom up, it's possible the GPU has calculated three or four frame already, and that you then see a strip of each of them. This is called screen tearing. Below you have an example of this, occurring at one instance. If your graphics card produces 200 fps and you have a 60 Hz screen, you could see three or even for of those lines. Nevertheless, many FPS prefer these types of artefacts over slowing down the game with VSync.
When you disable Vsync when you play on a 120 Hz monitor, you will experience less tearing. For example, if a graphics card produces an average of 200 fps (and each frame takes an average of 5 ms to produce), on a 60 Hz screen each frame (which takes 16.7 ms to appear) contains four or five underlying frames. That means that there will be three or four places with visible tearing. A 120 Hz monitor only needs 8.3 ms for compiling a frame, so here each frame will consist of two or three underlying frames, resulting in only one or two instances of tearing.
When you enable Vsync on a 120 Hz screen, you can experience a maximum slow-down of 8.3 ms before a finished frame appears. It's still perhaps too much for the most hardcore FPS gamers, but it won't bother most people. Stuttering occurs much less or not at all.
Gaming on a 120 Hz display without Vsync is what the participants in our test described as being more fluid and smooth, as each frame only takes 8.3 ms to appear which means that new information will be displayed quicker.
The negative side-effects of using Vsync (stutter and slowdown) are less with 120 Hz monitors, so even then you will experience a smoother game with the best possible quality thanks to Vsync, compared to gaming on a 60 Hz monitor.