This is basically a new window that contains HTML. They can be opened in many ways, but the following HTML is typical:
It would appear like an existing browser window, but may not have the address bar or other toolbars.
You would use the Watir attach method to access it, like so:
You can use a regular expression on URL or title like this:
Note: be careful to not assign the new window to your ie variable. Give the attached window variable a different name, like ie2.