What are the cathode rays? How are these rays formed?

Cathode rays are a stream of electrons that travel from the negatively charged electrode (the cathode) to the positively charged electrode (the anode) in a discharge tube.

Formation of cathode rays — When a high voltage (10,000 volts) charge from an induction coil is applied to tubes filled with gases at very low pressure (0.01 mm of Hg), the gases become good conductors of electricity and begin to flow from cathode to anode in the form of rays which are called cathode rays.