Sky, premul, key ??

What does the sky, premul and key buttons do? Please explain it to me.
Thanks in advance :slight_smile:

See my post here.

Very confusing. So how should I render my images? They all seem to have the same results. Sky renders the background color that is in the world setting. What is premul? I see no difference when I use photoshop. Same thing with key. How does premul with alpha? layers help me in compositing in photoshop?