Help me Understand Unemployment.

I am hoping someone tell how the unemployment rate is determined. In particular, when people leave the work force because they stop looking for work the unemployment rate goes down. When those same people start looking for work again the unemployment rate goes up.

My question is how is it determined that someone is looking or is not looking for work. I just do not know and was hoping someone could tell me how it is done.

Thank you for reading the blog. If you have knowledge of how this is done or how the information is gathered, please leave it in a comment to this blog.

Thanks form your time.