Please tell me details about "Vacuum tube " ?

Answer 1

Well, ya, tubes are switches, but not like "any other".
They are like transistors in that they can be turned on "a little".
All modern vacuum tubes are based on the concept of the Audion--a heated "cathode" boils off electrons into a vacuum; they pass through a grid (or many grids), which control the electron current; the electrons then strike the anode (plate) and are absorbed. By designing the cathode, grid(s) and plate properly, the tube will make a small AC signal voltage into a larger AC voltage, thus amplifying it. (By comparison, today's transistor makes use of electric fields in a crystal which has been specially processed--a much less obvious kind of amplifier, though much more important in today's world.)
The exception to this is the first type of tube called the "Fleming Diode" which was the predcessor to tubes as we know them, and had no grid. It's importance was that (like todays silicon diode/rectifier) it allowed current flow in only one direction.
In a CRT, the anode is the phosphorus coated glass, and the stream of electrons (or beam) is directed to it's destination on the screen by magnetic coils called a yoke.

Answer 2

There were many different kinds of "vacuum tubes" and they were replaced by the transistor.

A vacuum tube was generally used as a switch (or valve) that operated like any other switch.

Some vacuum tubes are still in use today in specialized applications.

A CRT (computer monitor, tv) are vacuum tubes.

Answer 3

Read: http://en.wikipedia.org/wiki/Vacuum_tube