Tabs keep you from having to open a new window/copy of your browser. I know Netscape and Firefox have them, and I think IE does, too. Not sure about the others. It opens a tabbed screen in the browser, and you can open up a separate page in it. You can open up several tabs at once. Look under your "File" menu, and you should see a command to open a tab, or there may be a sub-menu under "Open" if there's not an option in the main menu. I use Firefox extensions to color the tabs, which makes it easier to distinguish.

In Firefox, you just go to the "File" menu and then click on "New Tab." You can do that as many times as you need to.