A diagonal Spreads strategy is similar to a Calendar Spread, although unlike that strategy, the strike prices are different.
Bullish and Bearish diagonal Spreads can be constructed. Usually, the Short option Strike Price is more Out Of The Money than the Long Strike Price and these diagonals benefit when Volatility increases. Short diagonal Spreads are the inverse and the Short side is more In The Money than the Long, these positions benefit from a contraction in Volatility.
External LinksDiagonal Bull Call Spread
The Options Guide article on diagonal bull call spreads.Diagonal Spread
TastyTrade.com item explaining diagonal spreads.Option Spreads: Diagonal Spreads
Investoedia article about diagonal spreads.