The location of the supplier and the place of supply determines whether a supply is treated as an Intra State supply or an Inter State supply. Determination of the nature of supply is essential to ascertain whether integrated tax is to be paid or Central plus State tax are to be paid.
Inter-State supply of goods means a supply of goods where the location of the supplier and place of supply are in different States or Union territories.
Intra State supply of goods means supply of goods where the location of the supplier and place of supply are in the same State or Union territory.
Imports, Supplies from and to SEZs are treated as deemed inter-State supplies.
Source: CBIC Website