As far as I know, the difference between these two is seen in their intended use, price, and features,
RC quadcopters, often called Drones, are usually used for photography and videography. Their price can be relatively cheap and even expensive, depending on their features and capabilities. Some of their features are frequently used, such as cameras, GPS, and other sensors for navigation. Meanwhile, RC Airplanes are usually used for hobbies, research, surveillance, and even filming. The price of an RC airplane depends on size, complexity, and features, but it is generally cheaper than a high-end quadcopter.