While most commonly use to access information on the web, a browser can also be used to access information hosted on Web servers in private networks.
A browser window is the square that FireFox, Chrome, Opera, *shudder* Safari and Edge, draw the page in. If there's an iframe (a page inside a page, like what ads are held in), then those are technically windows as well. That's from an end-user perspective.
most commonly use to access information on the web, a Great Browser 9