I forgot what Alt + F4 does

I saw today an avatar on a forum which said "To view my display picture hold down Alt + F4".

Now, it was clearly some funny-man trick but then it occurred to me that I'm not certain what Alt+F4 does.

I've been using OSX for so long I had forgotten about Alt+F4 on Windows. I guess this is enough Microsoft-independence.

I never wanted to program on Windows or using anything Microsoft specific. This lead me to Java initially then Java on Linux and soon to Java on OSX...