What is the difference in a high def. tv and a plasma TV?

You can buy a HDTV that uses one of these different technologies:

CRT - like the old tube televisions

DLP - A projector bulb and a special chip with mirrors

Plasma - A glass screen with super-heated gas to create light

LCD/LED - A LCD panel like you find in the old calculators that sometimes uses LED lights behind it

The most popular technology is the LCD with the optional LED back lights.

Plasmas are less popular but with the few exception of the high-end LCD panels produce a better picture with a wider angle of view than most LCD units. But they are glass so they are heavy, consume more power and reflect lights like a mirror.

Hope this helps.

A Plasma TV is just a particular type of HDTV.

HDTV is just the general term of any TV that displays 720p or higher resolution.

Plasma TVs are just a special type.