Complicated questions: cookies, AJAX and more!

Let's say you were designing a system like microsoft passport. Here are my questions:

How would you make it so that other sites (retailers or whoever), would be able to login to your site, WITHOUT leaving their site. You would need to stay in their site, but somehow remotely query your DB's for account info. I'm not talking about iframes, but something more integrated. Basically like paypal or google checkout, but in the middle of the whole process. Is this where AJAX comes into play? I'm thinking my site would have some sort of API where it would return account info when queried, but how would I do that? Using AJAX? Or something else? I don't even no where to begin! I'll have more questions after this one is resolved (assuming it can be). Thanks in advance!

What - you mean like creating a web service? look into REST and SOAP if so.

I just thought I would move this to Programming Theory as you seem to be asking about the theory behind making a script instead of fixing a problem with a script you are making.

Andrew

If I were to do something like that, I would set up an LDAP server:

http://en.wikipedia.org/wiki/LDAP

It's great for maintaining a central pool of users, and the infrastructure is already set up to do just what you are talking about with LDAP queries.