When writing a letter, c/o simply means "in care of." It is used when a person receiving a letter does not normally get mail at that address. People commonly use it to send mail to themselves at a different address than usual or to get in touch with someone they do not have an address for.
TL;DR (Too Long; Didn't Read)
To send a letter in care of someone, start the address with the recipient's name, then write "c/o" and fill in the rest of the address.
What Does C/O Mean
Those who do not do a lot of formal correspondence may wonder what c/o means in an address. Simply put, c/o means that someone is "in care of" another party. It is a way to ensure that an item is not returned to the sender if the post office does not recognize the addressee as someone who gets mail at that address. Addressing a letter as c/o also ensures that the recipient knows that the letter is not intended for them, but that they are supposed to give it to someone else.
For example, if you wanted to write a letter to a traveling salesperson, but you do not know their usual address, you might write something to the company's home office in care of the salesperson's boss, who will know how to get in touch with him.
When to Use "In Care Of"
You may address a letter c/o when you do not know someone's primary mailing address, but you may also choose to send something in care of someone else if you are concerned about unwanted eyes or hands landing on the item. For example, if a construction worker lives in a bad neighborhood and does not want her packages to get stolen, she may have them delivered to her work address using a c/o address since she does not usually get mail at the office.
Alternatively, some people may want to ensure certain documents are not lost, so they may have them addressed to their attorney in care of their name, so the paperwork can be filed with other documents already in their lawyer's possession.
You may also use C/O if someone is staying in a hotel or on vacation at someone else's home.
How to Use C/O
To address a piece of mail to someone using c/ o, write the addressee's name, followed by their title, if applicable. Then you will add the c/o portion of the address, starting with "c/o" in lowercase letters and the person or organization you are sending the mail to.
For example, if you are sending a letter to someone at a hotel, address it:
c/o Marriott Hotel
123 Main Street
Los Angeles, CA 91234
If you are sending a letter to someone in care of another person in her company, use:
c/o Tina Fab
5678 G Street
New York, NY 12789