bt cotton is the transgenic plant which is obtained by introducing foreign genes that makes the resistance against

1. bacterial pathogens
2. insects pests
3. salt resistant
4. drought resistant

The transgenic plant bt cotton makes it resistant against the insect pests like moths, butterflies, beetles,flies etc.
Transgenic plants basically contain a foriegn gene which positively affect the plant.
Bt cotton contain that gene from a bacterium Bacillus thuringiensis (Bt) which naturally produce chemicals harmful to insects pests. So basically bt cotton is a insect resistant variety.

Bt cotton is the transgenic plant which is obtained by introducing foreign genes that makes the resistance against insect pests (2).
Bt cotton is a genetically modified variety of cotton producing an insecticide.(Bt) cotton was created through the addition of genes encoding toxin crystals in the Cry group of endotoxin.[3] When insects attack and eat the cotton plant the Cry toxins are dissolved. This is made possible due to the high pH level of the insects stomach. The now dissolved and activated Cry molecules bond to cadherin-like proteins on cells comprising the brush border molecules. The epithelium of the brush border membranes role is to separate the body cavity from the gut whilst allowing access for nutrients. The Cry toxin molecules attach themselves to specific locations on the cadherin-like proteins present on the epithelial cells of the midge and ion channels are formed which allow the flow of potassium. Regulation of potassium concentration is essential and if left unchecked causes death of cells. Due to the formation of Cry ion channels sufficient regulation of potassium ions is lost and results in the death of epithelial cells. The death of such cells creates gaps in the brush border membrane. The gaps then allow bacteria and (Bt) spores to enter the body cavity resulting in the death of the organism.