HDR stands for High Dynamic Range.  HDR for TV represents a major advancement in expanding the TV’s contrast ratio and colour palette to offer a more realistic, natural image than what’s possible with today’s HDTVs.  Experts believe that this feature is more important and noticeable than the resolution upgrade between HD and 4K.  In essence it improves the pixels rather than pumping through more.
What makes a HDR TV?
HDR:10 is the standard that is found on most HDR TVs.   The two most important factors that define how TV picture looks are contrast ratio, or how bright and dark the TV can get, and colour accuracy, which is basically how closely colours on the screen resemble reality (or whatever palette the director intends).
Contrast is one of the most important factors in how good a TV picture looks and it’s a key part of what makes a HDR TV. It refers to the difference between light and dark. The greater the difference, the greater the ‘contrast’.
There are two components:  peak brightness, which refers to how bright a TV can go, measured in what’s known as nits. The other measurement is black level which refers to how dark a TV image can appear and is also measured in nits (e.g 0.4). Most content for average TVs have a peak brightness of 400 nits. HDR: 10 increases brightness to 1000 nits.
The difference between the peak brightness and black level is known as the contrast ratio. HDR TVs have to meet specific standards for peak brightness and black level which helps give them the dynamic appearance.
This second of the most important aspects of HDR is colour. A TV must be able to process what’s known as 10-bit or ‘deep’ colour. 10-bit colour equates to a signal that includes over a billion individual colours/shades.  In comparison, standard TVs and Blu-ray uses 8-bit colour, which amounts to around 16 million different colours. With 10-bit colour, HDR TVs will be able to produce a vastly expanded range of colour shades, reducing overtly obvious gradations between shades and making scenes look far more realistic.
There are two formats; HDR: 10 (discussed above) and Dolby Vision which at the moment is less common but with a more impressive specification.  Dolby Vision format provides 12-bit colour standard which can provide 68 billion colours (which is the colour depth used in commercial theatres).  Dolby Vision can also offer up to 10x brighter pixels than HDR:10, of 10,000 nits,  although no TVs support this brightness level yet.  Dolby Vision requires a dedicated chip to function which must be integrated into TV – the only manufacturer to support this format currently is LG.
So if I have an HDR TV, will everything I watch will be in HDR?
There are two parts of the HDR system: the TV and the source.
The first part, the TV, is actually the easier part. To be HDR-compatible, the TV should be able to produce more light than a normal TV in certain areas of the image.  Tied in with HDR is wide colour gamut, or WCG.  The TV should have the Ultra HD Premium label to be deemed compatible.
Of course, making TVs brighter and more colourful costs money, and some HDR TVs will deliver better picture quality than others. Just because a TV is HDR-compatible doesn’t necessarily mean it’s going to outperform non-HDR TVs. The only thing the HDR label really means is that the TV will be able to display HDR content of relevant movies and TV shows.
The content is the hard part, which is gradually coming.  To truly look good, the HDR TV needs HDR content.  You have two options: UHD discs (with HDR) or streaming services. 
To play a UHD disc you need to buy a new Ultra HD Blu-ray player, there are currently only three on the market in the UK, one of which is the new X Box One S.   There are only about 20 movie titles currently available to buy although the list is growing fast.
TV shows and movies in UHD/HDR are now becoming available online via steaming services such as Netflix and Amazon Instant Video which can be viewed via dedicated TV app or the latest Amazon Fire TV box.  You will need a good broadband connection as this requires even more bandwidth !
Some of the latest games will also incorporate HDR content for releases on the new X box One S and the upcoming PlayStation 4 Pro.
Cables and connectors
You won’t need new cables for HDR.  Current High-Speed HDMI cables can carry HDR.  However, the source device (e.g 4K Blu-Ray player) and the TV must be HDMI 2.0a version to transmit the metadata.  If you have an AV receiver and want to use it for switching, it will also need to be HDMI 2.0a compatible.
If you put two TVs side by side, if one has a better contrast ratio and more accurate colour, and the other just has higher resolution (more pixels), the one with greater contrast ratio will be picked by pretty much every viewer.  It will look more natural, “pop” more, and just seem more “real,” despite having lower resolution. In other words, a 1080p resolution TV with excellent contrast and colour actually beats a 4K resolution TV with average contrast and colour every time.
HDR expands the range of both contrast and colour significantly. Bright parts of the image can get much brighter, so the image seems to have more “depth.” Colours get expanded to show more bright blues, greens, reds and everything in between.  HDR offers a Wide colour gamut (WCG) that enables more colours which were impossible to reproduce with TVs up to this point.

So should I buy an HDR TV or not?
If you want to future proof your purchase and get the best picture possible then HDR is certainly worth having and significantly improves image quality for all TV sizes, whereas 4K resolution is only more evident on larger screens.
Now that there’s an official HDR standard, in the form of Ultra HD Premium make sure you buy a TV with the Ultra HD Premium tag