When addressing a letter in care of is that the person you want to receive the letter or the person residing at that address?

The C/O is the person residing at the address.

Example:

John Smith

C/O Pocahontas Rolfe

1234 Notta Real St.

Jamestown, VA 58401

The letter is being mailed to Pocahantas' address, but is intended to be given to John Smith. It is in her care to give to him.