CGST, SGST and IGST, what's the difference?

GST in India is one tax collected in different components depending on whether a sale is within a state or across states.

Intra-state sale → CGST + SGST

When the supplier and buyer are in the same state, GST is split into CGST (central) and SGST (state), each at half the GST rate. For 18% GST that's 9% CGST + 9% SGST.

Inter-state sale → IGST

When the buyer is in a different state, you charge a single IGST at the full rate (e.g. 18%). The centre later apportions it with the destination state.

What decides it: place of supply

The place of supply, generally the buyer's location, determines intra- vs inter-state. That's why your invoice needs both your state code and the buyer's. Lekhae works this out automatically from the two state codes.