If you retire from active service, you collect a pension. There is a catch...if they need you, they can recall you back into active service. You then have a choice, go back into active service or give up your pension. When I was in Iraq in the late 2000's I ran into a Army Colonel that looked like he was in his late 60's. I asked him why he hadn't retired. He was called back into active service because he had some special technical skills that the Army needed. They wanted him to train other soldiers.