First of all, LED is not really a display technology unless you are talking about an LED digital sign where the pixels are comprised of actual LEDs. What manufacturers are referring to with LCD LED monitors/TV’s is that the back lighting is LED. With a LED back-light grid, the TV can be dynamically lit or dimmed based on the colour of the pixel in a specific area. Simply put, this lets LCD TV’s produce blacker blacks as the back-light is essentially turned down behind the pixel areas that are black on the image.
After saying all this, the traditional difference between LCD and Plasma was the ability to produce true blacks. Think of Plasma as a miniature LED sign, with the light being produced at each pixel. LCD uses back-light which shines through a filter to produce a pixel on-screen. (similar to LCD but each pixel has it’s own RGB grid)
Plasmas tend to run hotter, and do still have some burn in issues with static content, although this has been improved with advanced design and settings that can add some movement to the screen occasionally. You should refer to manufacturers on how to set this up on Plasma screens you are installing to avoid the burn in from happening.
LCDs do have a bit of “image retention” with static content, but this tends to go away when the power is cycled.