How can I get a Gmail appearance such that all of the mails in my email tabs (Inbox, Sent Mail etc.) are visible in one window? I should be able to see all of my emails in one big window, without having to click on tabs on the left hand-side. I know it is possible because I have seen my friend do it, but I don't know how!
If I understand correctly what you want to accomplish, you should set your inbox to default and disable the categories tabbing.
That’s for an all-incoming-mail-in-the-same-window inbox. Otherwise, if you want to see absolutely everything, including chats, archived emails and what’s under your labels as well, go to All Mail from the left-hand side menu (under Inbox, Sent Mail etc.).
That's probably the "All Mail" item.
"All mail" shows all of your mail messages, no matter what their label(s) might be1 except for the message in your Spam and Trash buckets.
1 "Inbox" is just a label, albeit a special one.