Hii,

A cathode ray tube consists of five major parts: an envelope or container, an electron gun, a focusing system, a deflection system, and a display screen.

The cathode , or electron emitter, is made from a cesium alloy.

Cesium is used as a cathode in many electronic vacuum tube devices because it readily gives off electrons when heated or struck by light.

In a CRT , the cathode is heated with a high resistance electrical wire.

Or you can define it as

The cathode - ray tube ( CRT ) is a vacuum tube that mainly contains one or more electron guns and a phosphorescent screen, and is used to display images.

It modulates, accelerates, and deflects electron beam(s) onto the screen to create the images.

Hope this will help you,

All the very best!!