Radar includes Primary Radar and Secondary Surveillance Radar. ADS-B and Radar are quite different in all ways. Below is just a brief description of the difference, not quite precise, but enough for basics.
1. The ways to get positions are different. Radar calculates aircrafts positions based on the time radio signal is sent and received. While ADS-B uses airborne GPS to get aircrafts positions.
2. Requirements for airborne equipment are different. Primary radar doesn’t require airborne equipment. Secondary radar requires airborne transponders. ADS-B requires airborne GPS receivers and ADS-B transmitters.
3. Prices are different. Radar is expensive, while ADS-B ground stations are cheap. Though the prices may vary on different brands and different locations, radar is at least ten times more expensive than ADS-B ground stations. But this price is just for ground part. The airborne part price is far more complicated and can’t be explained clearly in short words.
4. Requirements for maintenance are different. Radar is bigger, more complex and requires harder work for maintenance. ADS-B ground station is smaller, easier and requires much less work for maintenance.
5. Applications are different. The main applications for radar and ADS-B are ground-to-air surveillance and airport surface movement surveillance. ADS-B can also be used in air-to-air applications.
6. Coverage areas are different. Years ago, radar and ADS-B coverage are almost the same. Then with the development of satellite based ADS-B, ADS-B coverage expands to all the globe, while radar is still the same as before, only can cover areas where radar can be installed.