Dry coolers are heat-transfer devices that use air to remove excess heat into the atmosphere. Usually, a dry cooler presents a heat exchanger (it can be either microchannel coils or a finned tube) and fans. The fans are responsible for directing the air flow through the heat exchanger. Here is where the fluid (it can be water or another solution) is cooled and then circulated back into the system.
Clearly, for a dry cooler to be efficient, the air temperature needs to be cooler than the water (or any other glycol solution) in the system. This explains why, for example, dry coolers work better in cold climate regions (while wet cooling towers are more efficient in hot and dry climates). Dry coolers tend to not be affected by air humidity, on the contrary of wet cooling towers.
Dry coolers have a limited cooling capacity, compared to wet cooling towers: the first one only brings the water temperature down to the ambient temperature (additional cooling can be achieved in other ways), while the latter can bring the water temperature up to 5 degrees down the ambient temperature.