Sometimes you need to send a letter or package to someone and you either don't know their current address or you know they're staying somewhere where they do not normally receive mail. So what do you do? That's when you take advantage of c/o to send your mail to that person "in care of" another recipient.
TL;DR (Too Long; Didn't Read)
To use a c/o address when sending mail, simply write the addressee's name and then write "c/o" and the name and address of the person who you are leaving the letter or package in care of.
Meaning of C/O
Mail sent with the letters c/o in the address are sent "in care of" someone else. That means that the post office should deliver the mail to the person or entity such as a business or company listed "c/o" on the address, who should then give it to the person to whom it is addressed.
Why Use "In Care Of?"
Generally, using c/o is a way to ensure that the post office does not return an item to the sender when they see that it is addressed to someone that does not reside or work at a given address. But addressing something in care of someone else also lets the person that receives the package know that he or she is supposed to pass it along to the proper recipient.
A letter or package may be sent in care of for a variety of reasons, but most often it is either because the mail was sent somewhere the addressee does not usually receive mail, or because the sender does not have a current address for the recipient but does know the address of someone who might know that person's address. For example, if a salesperson is out of town for a convention and is staying at a hotel, the sender may choose to send their mail to the salesperson in care of the hotel. Or if an employee recently was transferred and the sender does not know the office the employee was transferred to, the mail could be addressed to the employee's previous manager as she would likely know where the employee is now located.
Aside from those two examples, people may sometimes send packages c/o when they do not trust it to be properly delivered at the recipient's normal address. For example, Company X and Company Y have a bad business relationship and John left Company Y to work at Company X. John knows Tom from Company Y instructed his receptionist to immediately destroy all correspondence from Company X, so he may address his letter to Tom in care of Tom's coworker Wendy, who he knows will personally hand the letter to Tom.
Alternatively, if a woman wants to send her grandson cookies but she knows that he has had a problem with his mail getting stolen, she may address the package to him at his office, even if he is a plumber who spends most of his time out on calls. Since she knows he doesn't usually get mail at the company, she might send the package in c/o the company address to ensure the post office does not return it to sender.
C/O Address Example
If you are sending something in care of someone else, be sure to write c/o in lower case. Format the address this way if you are addressing it to an entity:
c/o Business Company
3494 C Street
Random City, Nevada 49895
If you are sending the letter to an individual, it should be sent this way:
c/o Wanda Peter-Deetz
5555 House Address Lane
Small Suburb, Arizona 25979