LCD: LCD HDTV's use fluorescent light bulbs and liquid crystals to light up your screen. While these TVs have had their issues with viewing angles, motion detail, and black level creation, those issues have largely been dispelled with more modern technology. LCD TV's have a matte plastic screen that greatly reduces glare, and are very energy efficient.
Plasma: Plasma HDTV's use polarized gas particles to light up your screen. While many believe their picture quality, viewing angles, and motion detail are better than that of LCD HDTV's, they are less energy efficient, and can produce more glare in rooms where there is a lot of ambient light.
LED: LED HDTV's are LCD TV's, but use LED lights instead of fluorescent lights to send light through the liquid crystals. LED HDTV technology brings a nice feature set to the mix. These TVs create very dark black levels (rivaling Plasma), but keep the brightness of an LCD. LED TVs offer greater energy efficiency than plasma, LCD and DLP. Most will reduce glare just as LCD TVs do, but some have implemented a gloss front (similar to Plasma) to increase the contrast ratio, and produce a deeper black. 'Side-Lit' LED TVs are also extraordinarily narrow when compared to Plasma and traditional LCD TVs, some as narrow as 1.2".
DLP: DLP TVs, or Digital Light Processing TVs, are the last remaining technology of rear projection TVs available in mass production. These TVs use a halogen light source that gets filtered through a 'color wheel' to create all the colors in the spectrum. You cannot hang most DLP TVs on your wall as they are around 15" deep. These TVs lack viewing angle, and brightness when compared to flat panel TVs like LCD and Plasma. However, you can get a whole lot of screen size for not a lot of money.