Direct evaporative cooling adds moisture to the air stream until the air stream is close to saturation. The dry bulb temperature is reduced, while the wet bulb temperature stays the same.
Indirect evaporative cooling does not add moisture to the primary air stream. Both the dry bulb and wet bulb temperatures are reduced.
During the heating season, an indirect system’s heat exchanger can preheat outside air if exhaust air is used as the secondary air stream.