Postal Letter

What type of mail is considered certified?

I just posted a question about the mailman stopping by and hand delivering a letter. I was in a towel and could not answer at the moment but when he was leaving I saw him carrying a letter. Pretty much all of the responses said that it was some type of certified mail. Which leads me to my next question, what type of mail is certified? Sorry, I know this is a dumb question but I'm a natural worrier.

Public Comments

1. Mail that's been certified from the sender. i.e. something important like a paycheck , official documents... The mail carrier will just leave a slip on where to pick it up.
2. anything that the sender whants to be sure that you or whoever gets could be from a law firm or could be from Ed mcmahon
3. Any piece of mail that needs official documentation that it has been received by the addressee. It could be anything from a check, a late notice or official documents from a lawyer (such as a divorce decree). In other words, it just shows that you received it and signed that you received it.