Certified mail is an
ancillary service the
United States Postal Service provides as a legal means to determine that an article of
first class mail or
Priority Mail was received at its intended destination. This is not to be confused with a
certificate of mailing, which simply states that you mailed a given article of mail with a clerk.
An article that is mailed certified is handled much like any other item of mail, but there are a few differences. For one thing, a number is assigned to it for internal tracking purposes (you can't (yet) track a certified article on its course), and on arrival, it must be signed for. Oftentimes a return receipt is affixed to the article, and it as well must be signed by the recipient to be returned to the point of origin. With this comes an option of restricted delivery, which for an added fee dictates that only the addressee or an appointed agent of the addressee can sign for the mailed article.
If somebody sends you a certified letter, read it. It's probably something that will affect you if you don't.
If you are sending something via certified mail, it's probably going to a collection agency or the IRS to prevent them from doing a denial of receipt act.
Current fee for this service is $2.30.