IFRAME control is simply a way to embed another browser window inside the current one. That's all.
Your login issue witrh IE is caused by the fact that your browser is set not to accept 3rd party cookies. If you change that to allow it, you'll be able to login. However, this would be a bad idea for a pulic facing website - you never know what kind of security settings your visitors will have...
As far as QueryString paramters, it's the same as in your browser's address bar. Except it's inside a tag...