Tonight is the evening that happens on the current day—not last night, not tomorrow night, but tonight. If your favorite TV show is on tonight, you'd better hurry up and finish your homework so you can tune in.
If your friend suggests seeing a movie tonight, she means the upcoming night, the one that immediately follows today. Prior to the 18th century, tonight was two separate words, to night, and then until the 20th century, it was hyphenated: to-night.