Many older refrigerators and most small refrigerators (like small bar and dorm refrigerators) do not have fans, but most modern frost-free refrigerators have two. One is under the refrigerator to cool the compressor and force air through the exterior coils. The second is inside and moves air around the coils inside the refrigerator. This second fan helps provide more even cooling, and also aids in the defrost process.
See How Refrigerators Work for complete details on the inner workings of refrigerators.
Here are some interesting links: