OK, I have a friend who is a student and would like to visit me here in the u.s. during summer vacation. I understand there is an invitation letter I need to provide my friend to facilitate in obtaining a visitor's visa. What does this letter need to specify? And what other information can you provide me for this particular matter? Thank you.

This is confusing to me, and I've subscribed to this thread because I'd love to see what an expert in visas and immigration law says.

Having said that, I don't see why your friend would need a visa. A passport yes, and plane ticket. But if he/she is only staying for the summer (2 months) why the need for a visa? A return ticket for a specific date should be enough. It's called a vacation.

But that's just my take on what you've written. I'm anxious to find out what the experts say, because this makes no sense to me, unless there's something you're not disclosing.

Do you mean a J-1 visa?
J-1 Visa (http://j1visa.state.gov/)

I can't provide a link from my iPad but there are specific matters you need to address in the letter of invitation. Such as guaranteeing that the applicant will have a place of residence, enough money to cover the stay, etc. this letter is compiled and sent to the applicant, not to Immigration. Please Google 'letter of invitation for US visitor's visa' to view guidelines.