LCD TV: CCFL
Until recently, all LCD TVs were backlit by always-on, CCFL (cold cathode fluorescent) lamps. This ageing technology has been superseded by the superior LED method on more expensive sets, but is still standard on some cheaper models.
LED TV: Direct LED
These displays are backlit by an array of LEDs (light emitting diodes) directly behind the screen. This enables localized dimming – meaning immediately adjacent areas of brightness and darkness can be displayed more effectively – and greatly improves contrast. LED TVs are also more power efficient and capable of a wider colour gamut than CCFL sets. Because of the extreme cost of mounting these arrays of LEDs, Direct LED TVs have largely been out muscled by Edge LED.
LED TV: Edge LED
With these TVs, LEDs of the backlight are mounted along the edges of the panel. This arrangement enables radically slender displays and offers superior contrast levels to CCFL, but can't achieve the same picture quality as directly lit LED sets. However, they do come in far cheaper which is why most LED TVs out there now use this technology.
The backlighting on OLED (organic light emitting diode) sets is achieved by passing an electric current through an emissive, electroluminescent film. This technique produces far better colours and higher contrast and also enables screens to be extremely thin and flexible. This is the holy grail display technology and only in 2014 did a big screen OLED TV go on sale. So it's brand new, it's expensive and the top brands are still struggling to get their heads around it. To date, only LG has been able to release full sized OLED TVs.
PDP (plasma display panel) TVs use glass panels containing millions of tiny cells filled with a mixture of inert gases. Electricity excites the gases, causing them to illuminate the pixels across the screen. Plasma, while arguably superior to LCD in terms of contrast and colour accuracy, is only viable on large (42in+) screens and has been dropped by all but a handful of manufacturers. You'll be lucky to find one on the shelves these days.
Some manufacturers are now making TVs that have slightly curved screens. But unlike old CRT TVs, the curve is inwards rather than outwards. The idea is that this makes every pixel equidistant from your eyes, delivering a more satisfying picture. However, there are drawbacks for this type of screen - the main one being that if you sit far enough to one side – more than 40 degrees or so – the curve clearly starts to affect the image's geometry, foreshortening content near to you and compressing the image's center.