LEGAL NOTE: I do not condone pirating and emulating video games unless it is the only accessible way to play them. Physical-only games for out-of-production consoles do not count as accessible. I am resharing these links for archival purposes.
Jet Set Radio is an arcade skating game similar to Tony Hawk’s Pro Skater and other adjacent games. You score points by doing tricks, as well as tagging graffiti. Each level has a set objective you have to complete on a timer - usually completing all tags marked in RED, or tagging all enemies on the map.
Don't think you're all alone, though! After completing the required amount of tags on a given stage, a hazard will be deployed; this can range from a single armed officer to whole military squads. These add much needed difficulty to the later stages.
While progressing through the story mode for the first time, you will be occasionally interrupted by challengers; after completing the challenge, they will join your team as a playable character. The challenge usually consists of copying the character's tricks. In the case of Yo-Yo and Combo, this challenge is a tag race.