IOS Foreground and background interaction

Make a landing interface on IOS. I don't know how to describe my problem, uh. Similar to the QQ landing interface type, I put the user entered the user name and password, to the background, so that the background to determine the correct user name and password, and then pass a value to me, tell me the user name and password is correct, user name and password correctly landed success, on the contrary landing failure .... Now the problem is, I do not know how to pass the username and password to the background?? Do not know backstage how to pass something to me?? Can you give me a detailed explanation about this aspect? or give me some keywords? or a connection? Thank you. What's the front desk? My understanding is that the front desk is the iphone interface, what the user sees. Backstage is some technical support,