Tab functionality ala your web browser is provided by an external package
tabbar-mode. The source code is on github, and there is an extensive page about customizing it on the EmacsWiki. In fact the first Google Search hit for "emacs tabs" points to the EmacsWiki page. It's also available as a package via Melpa.
Once installed you can toggle it on or off with
M-x tabbar-mode. To turn it on at startup you can put the following in your init file.
If you're using Aquamacs, it's enabled by default and you can disable it at startup by putting the following in your init file.
(when (featurep 'aquamacs)