GDP is the total dollar value of all final goods and services produced within a country's borders during the fiscal year. Let's look in on Professor Max E. Mizer's classroom where he's teaching Econ 101.
Professor: Any Questions? Yeah, Trash.
Trash: What if I like buy a used car? Does that count?
Professor: No, the car was counted when it was made. Yeah, ESPN.
ESPN: What about the man who works at home? Does he count?
Professor: Hey, you hit a homerun with that question. No, his work doesn't count. Economists like to say 'if you want GDP to go down, marry your house keeper.' Yes, ESPN.
ESPN: What I mean is, the Man who works at home.
Professor: Again, if I paint my own home, it's not counted in GDP.
ESPN: Man, you're really striking out on this question. I mean, the umpire.