First class mail is the standard service that all customers of the United States Postal Service
are familiar with. You put a thirty-nine cent postage stamp
on your envelope (assuming you're mailing within the United States), find a mail box
to put it in or give it to a postal clerk
, and the postal service happily delivers it to its intended destination, if it doesn't get lost on the way.
It's how you send your tax returns to the IRS, it's how you send your christmas card to Grandma, and it's how many people still send their check to their utilities - and even then, it's how the online bill payers send your money to the people you owe money to.
It's how your postcards get to their intended destination - sometimes after you have arrived home. =)
Depending on where you are located, it usually takes anywhere from one to three days for an article of mail to get from point A to point B, with slight variances in time owing to any number of factors - holidays, weather, lunar phase, Sundays (or Saturdays if you're in a part of town largely inhabited by Seventh Day Adventists), etc. If your intended destination is within 75 miles of where you dropped your letter, though, the odds of the letter getting there the next day are very much in your favor.
Thing with first class mail too is that it is highly flexible in terms of services available. More than 500 items sent in one shot, assuming you have sorted your items and met certain conditions, can garner you a per-item discount between two cents and twenty cents by way of bulk mail. Send it certified mail, or send it registered mail. Insure a set of plane tickets as you mail them to a friend. Use C.O.D. to get the USPS to collect money from somebody who owes you.
It's also necessary for dealing with legal means at this time. To this day, most collection agencies (for instance) won't accept requests verbally under any circumstances for legal reasons, and God forbid you send one to them via email.
As of the date of this write up, cost of this domestically is $.37 for up to one ounce, plus $.23 for each additional ounce, up to maximum 13 ounces; after that, it becomes priority mail. Postcards are always $.23. Bulk mail rates are even more variable, and can be reviewed on Notice 123. International postage is $.50 per ounce to Canada or Mexico, and $.80 per ounce to any other country.