I've read a lot of posts about status on different programs, but I'm confused. Being a newbie to this world, I don't know which is true, someone help?

1. Status (such as silver, gold, etc.) is earned once through a particular amount of mileage flown
2. Status is earned once and then maintained through minimum mileage requirements each year
3. Status has to be re-earned every year

Which is true? Or does it work some other way?

Thanks!