What is the difference between DataSet and DataReader?

Short answer: And forth between rows (using DataRow and DataColumn). DataReader: A DataReader is a forward-only, read-only data cursor.

Real-world example (ShopNest)

For large order exports, ShopNest uses SqlDataReader (forward-only, fast). DataSet is heavier and mainly for disconnected editing scenarios.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.