C/O stands for "in care of." So when you address the envelope you put the name of the person who is to receive the mail first and the second line is the business, building name, etc. that the mail is sent in care of. It also helps the post office in case an error is made in the address when writing it out. They'll know to try to find the business the envelope is being sent to and not the individual. So always put the "C/O" on there. Always.

Yep, as johnnylightning says, makes no difference. If I'm sending via venue I will put on c/o and the name of the production, in case it gets picked up by someone who may not have read the posters outsiide, but otherwise go as the mood takes me.

It makes no diff if you put c/o or not it will still get there. I generally put the persons name and then c/o before the agency name but you dont have to.

if u find a fan mail and it says in c/o should you put that one there it? my friend found me a fan mail but it did not have in c/o untill he found a other do u think it will still get there?