“5G” is a marketing umbrella term. It refers both to the next generation of LTE (Long Term Evolution, the incremental improvements that have been added to 4G) and also New Radio, a new cellular protocol which uses new portions of the radio spectrum.

The features of 5G are higher bandwidth (especially in situations with high interference/poor reception), a higher density of users supported (up to 100k users/per square kilometer iirc), and better performance at high speeds (e.g. on bullet trains).