Rewards allow a player to gain extra ships or cards and provide an incentive to ally with the defensive side of an encounter. If the defense wins an encounter, each defensive ally can draw one card from the deck or retrieve one ship from the warp for each ship the player sent to help defend. If playing with the reward deck, a player may also draw cards from there instead of from the cosmic deck. A player can take a mix of rewards, e.g. if entitled to three rewards, a player can draw one card and retrieve two ships; but the player must decide what all of his or her rewards will be before drawing any cards.
The Fantasy Flight Games edition of Cosmic Encounter sometimes refers to rewards as "defender rewards" or "defensive ally rewards." These terms all refer to the same thing; the use of multiple terms to refer to the same game mechanic is likely an oversight on FFG's part, and is redundant because some cards and powers allow players to receive rewards even if they are not defensive allies.