What is the Refresh Rate?
The refresh rate is the number of frames per second the television can display. Its unit is a Hertz, which is equal to 1/second. 120Hz means it can draw 120 images per second.
The refresh rate of televisions is misleading and is mixed with marketing lies. The increased refresh rate (120Hz and higher) was introduced by manufacturers as an indirect way to reduce the motion blur problem of LCDs and LEDs. However, the response time of 120Hz and 240Hz TVs is usually the same as 60Hz ones.
Additionally, manufacturers are inflating the advertised refresh rate number like with Samsung's Clear Motion Rate as explained in the next page of this article.
To prove that a 120Hz TV does not reduce motion blur, we filmed in slow motion a 60Hz TV next to a 120Hz one when displaying a Blu-ray movie. As you can see, the amount of blur is exactly the same.
The same is true for video games. Video consoles games (even the Xbox One and PS4) displays at maximum 60 frames per second. When displaying this on a 120Hz TV, the panel just draws each frame twice, so it doesn't reduce the blur.
Does the refresh rate mean the same thing for an LCD, LED and Plasma?
No. An LCD/LED panel retains the picture for the entire frame duration. This is called the sample and hold method. A plasma TV works differently. It uses very short pulses to draw the picture. For each frame, it emits a series of pulse depending on the intensity of the colors.
When LCD TVs started advertising higher refresh rates meant to reduce motion blur, plasma TVs manufacturers didn't want to appear obsolete even though they display better motion by default. To keep up with their marketing, they started including a number in Hz. This number is not the amount of frames per second it can display, but the inverse of the duration of a small pulse. For example, a 600Hz plasma means its pulse length is 1/600 second, even though it only draws 60 frames per second.
Displaying a 120fps signal on a 120Hz+ television
As of 2013, there is no television officially supporting a true 120 frames per second signal as an input. Even a real 120Hz or 240Hz television does not support a source of 120fps. They can only display at that speed frames created by themselves using motion interpolation. For example, you can feed a television a 1080p @ 24Hz signal, which it will up convert to 1080p @ 120Hz internally; but you cannot feed it directly a 1080p @ 120 signal.
Despite the fact that the HDMI version 1.4 added support for 1080p @ 120Hz, there is no content available to display on a TV at that frame rate. Only a computer can generate this high frame rate. This will not change anytime soon. Gaming consoles are limited to only 60fps, even the PS4 and Xbox One. Movies are being shot at 24 fps. The first movie shot in 48fps is The Hobbit and it is only available at that speed in some specific theaters.
For TVs, the refresh rate means nothing, especially with manufacturers inflating their numbers. A high refresh rate TV does not guarantee it to be motion blur free. Even for console gaming, there is no advantage to a 120Hz+ TV because the console video games are capped at 60 fps anyway.
For gaming on a PC though, a higher refresh rate monitor is definitely worth it if you have a strong enough graphic card.
The next pages of this article explains two features usually found on 120Hz+ TVs, but they are irrelevant to gaming.