What is the difference between Nifty spot and Nifty futures?

As expiry approaches, futures price converges to spot. This convergence is called "basis decay" and can be exploited with calendar spread strategies.

Nifty spot is the actual index value (calculated from 50 stocks). Nifty futures is a derivative contract for future delivery — it trades at a premium (usually 50-150 points above spot) due to carrying costs.