Antenna diversity typically uses two antennas in order to increase the quality and reliability of wireless communications. In order to understand diversity antennas, you need to understand the idea of multipath interference. Basically, multipath interference is caused by radio waves bouncing off of different objects on the way to their destination.
When these radio waves arrive, they have the potential to interfere with each other. Diversity antenna capable devices support multiple antennas (usually two) in order to combat this phenomenon and minimize its effects. While diversity antennas are not required for 2G/3G connections, they are highly recommended in order to get the most reliable connection, especially in areas of low coverage.
Identical or highly similar antennas should also be used for the best results. On the other hand, 4G connections use MIMO communication (multiple input multiple output) and therefore should always be used with 2 antennas. As a result, the second antenna used with a 4g modem is not considered a diversity antenna.