Bt cotton is

A transgenic crop is a crop whose genes have been altered by manipulation.
This crop is known as genetically modified crops (GM crops).
GM plants can reduce reliance on chemical pesticides (pest-resistance crops).
Transgenic means that one or more DNA sequences from another species have been introduced by artificial means.
Transgenic plants can be made by introducing foreign DNA into a variety of different tissues.
It is a plant that has a novel combination of genetic material obtained through biotechnology that adds a new quality to the parent crop like herbicide-tolerance. 

🔑Key Points:
Bt Cotton- 
Bt cotton is the only permitted GM crop in India.
In which the soil is obtained by hybridization of two foreign genes with the bacterium "Bacillus thuringiensis" (BT).
These genes help the cotton crop secrete a toxic protein to protect it from an insect called the common pink bollworm.​
Some strains of Bt produce proteins that kill certain insects such as lepidopterans, coleopterans, and dipterans.
Bt forms protein crystals during a particular phase of their growth.
These crystals contain a toxic insecticidal protein.

🔴Additional Information:
Golden rice-
Rice is a staple food in many countries.
By inserting two genes from daffodil and one gene from a bacterial species into the rice.
Scientists produced rice capable of synthesizing β – carotene.
Tomato-
Transgenic variety of Tomato – Flvar Savr.
Due to inhibition of polygalacturonase enzyme which degrades pectin.
So tomato variety remains fresh.