In a tft screen of computer what is the abbreviation of tft.tft stands for what?and how does it function compred to a crt monitor?

TFT stands for "Thin Film Transistor". With TFT LCD monitors, each pixel is controlled by one to four transistors to create the required colour.

As for how they work, well I'll just let you read that yourself:
http://en.wikipedia.org/wiki/Cathode_ray_tube
http://en.wikipedia.org/wiki/LCD