Describe Nil rated and zero rated tax

Nil Rated It is the supply of goods and services that have 0% GST.
Example: Grains, salt, jaggery etc.

Exempted It is the supply of goods and services that does not attract GST and allows no claim on ITC.
Example: Bread, fresh fruits, fresh milk and curd etc.

liquor, petrol and deisel nil rated/0% rated/non gst

Nil Rated Supply -  Nil rated supply is the supply which is chargeable to nil rate of tax or 0% of tax. So goods or service which are said to be covered under nil rated supply are the ones which are chargeable to 0% GST.

Zero Rated Supply - There is a common understanding that supplies liable to 0% tax rate are the zero rated supplies which is wrong.  In GST, Zero rated supplies are the ones which are executed under Section 16 of the IGST Act. It includes :

Export of goods or services or both

Supply of goods or services or both to a SEZ developer or SEZ Unit

Exempt Supplies - Exempt supplies are the supplies which are wholly exempt from tax under GST Act. 

petrol diesel.is non gst product

Yes, you are right.
Sales tax on petrol diesel is one of the major revenue sources of State Govts. These State govts are reluctant to get into new GST. Hence petroleum products are not brought under GST.