The aim of interdelivery spreads is to bet on the price difference of the two contracts. Depending on the concrete underlying investment, one anticipates that the price difference between both months either widens or narrows.
For example, if a trader is long June corn and short August corn, then the trader anticipates that the price of June corn would increase and the price of August corn would decrease; therefore, the gamble is on the widening of a price difference.
The two most famous types of interdelivery spreads are bull spreads and bear spreads. In a bull spread you long the nearer contract and short the more distant one. The strategy name is due to a universal rule for numerous storable commodities, such as corn and pork bellies.
In a bull market, the near contract will increase over the distant months. In a bear spread you do the reverse, you sell the near future and offset it by purchasing a future with an extended delivery date.
Other examples of interdelivery spreads would be to go long on a crude oil futures contract with delivery next month and selling short on the same contract where delivery takes place in 6 months. Spread traders are merely interested that the long positions they hold increases in value against their short positions.