Tough to nail down, as everybody has their own personal interpretation of the norm; however, in general terms, as defined in part by Wikipedia, it can be said,
[Tuners are] "Japanese vehicles, such as a Honda Integra DC5, Mitsubishi GTO, Toyota Supra, Nissan Skyline, Mazda RX-7, Subaru Impreza, and the Mitsubishi Lancer Evolution series. These cars are most commonly modified with the more expensive mods available. The most popular modifications include suspension upgrades, exhaust systems, and turbos."
The purpose is generally to improve the car's performance, handling, presentation and/or improve and display the owner's style of appearance or driving.
Common modifications include racing-inspired paint, graphics, and appearance accessories such as aero-kits, bumper covers, spoilers, splitters, whaletails, vents, and scoops. Most modifications are in the nature of bolt-on components.