What Is the Difference between Volatility Skew and Volatility Smile?

Volatility skew describes a pattern where implied volatility changes monotonically across different strike prices (e.g. higher IV for lower strikes). Volatility smile is a U-shaped pattern where both very low and very high strike options have higher implied volatility than the at-the-money option.

The smile is essentially a skew that is symmetric around the ATM strike.