The Seventh Schedule: three lists, and what happens when a Union law and a State law collide
Background
The Seventh Schedule of the Constitution contains three lists: a Union List, a State List and a Concurrent List. These lists set out the subjects on which Parliament and the Legislatures of States are empowered to make laws. Parliament is competent to make laws on matters enumerated in the Union List, and State Legislatures on matters enumerated in the State List. Both the Union and the States have the power to legislate on matters in the Concurrent List, and only Parliament has power to make laws on matters not included in the State List or the Concurrent List. In the event of repugnancy, laws made by Parliament shall prevail over laws made by State Legislatures, to the extent of the repugnancy; the State law shall be void unless it has received the assent of the President, and in that case it shall prevail in that State. Laws made by Parliament may extend throughout or in any part of the territory of India, while laws made by State Legislatures generally apply only within the territory of the State concerned. This is why provisions of law on matters in the State and Concurrent Lists vary from State to State.
In plain words
Seventh Schedule — The schedule of the Constitution that lists the subjects on which the Union and the States may legislate.
Concurrent List — The list of subjects on which both Parliament and State Legislatures may make laws.
Repugnancy — A direct conflict between a Union law and a State law on the same subject.