Also called CGST, SGST, IGST, UTGST, central GST, state GST, integrated GST
GST is one tax, but it is collected in parts. On a sale within the same state, the rate is split equally into Central GST (CGST) and State GST (SGST). In a union territory without its own legislature, UTGST takes the place of SGST. On a sale between two states, the full rate is charged as Integrated GST (IGST).
Charging the wrong type is a common mistake. If a sale should have been IGST but you charged CGST and SGST, the tax went to the wrong government and the buyer may not be able to claim it. Fixing it means correcting the invoice and the return.
A shop in Pune sells goods worth ₹10,000 at 18% to a customer in Nashik. That is within Maharashtra, so the bill shows CGST ₹900 and SGST ₹900. The same sale to a buyer in Indore is inter-state, so it shows IGST ₹1,800.