PLASMA monitors have a much superior picture quality than TFTs.. But most companies are deciding to stop the PLASMA Dispaly device production due to the very high prices and lesser demand. So you have to do some research.. I heard there is a lot of bargaining ongoing in the Plasma market.. check it out !
TFT is a special kind of transistor used mainly in LCD displays while Plasma displays uses noble gases that is turned into plasma by electricity.
The main difference between the two is offcourse the display panel as said by ED Williams. The other difference is that the picture quality of the plasma is far more better that TFT but the average life of plasma is very less than TFT.