Things that are tacky are cheap, flashy, garish, gaudy, loud, tawdry, or trashy. Tacky clothes are a fashion nightmare, and tacky comments are embarrassing for everyone involved.
If you wore a bright orange suit to school, with a neon green hat, you'd be dressing in a tacky way. Tacky clothes are usually inexpensive yet flashy and showy — obnoxious. When people make tasteless, trashy comments, it's also considered tacky. But if you're in art class and someone says that your paper-mâché project is tacky, don't get mad — they probably just mean that it's not dry yet and still sticky to the touch.