The Account and Options menus

 
The Account Menu is your email address in the top right of the Gmail window. If you click on your email address, a menu will appear with information about your account, a link to Account to change overall Google Apps settings, a button to Add Accounts to your current login session, and the button to Sign Out.

The Gmail Options menu is the cogwheel icon on the right, below your email address.  From this menu you can access your Display Density, Gmail Settings, Themes, the Google Apps Help Center, and send any feedback directly to Google (the Report a Bug link).  
 
Most of the changes to your Gmail account are under the Gmail Options > Settings menu.  Below is a brief description of each one.
 
  • General: Set a signature for your emails, set an away message, set your default text style, change your default display language, and more.
  • Labels: Organize your labels, Google+ circles, and show/hide your labels.
  • Inbox: Customize the display of your Inbox.
  • Accounts: Configure your email aliases and set your default From: (Send mail as) email address.
  • Filters: Add, modify, or delete your mail filters (mail rules).  You can also create new mail rules from the Gmail Search window at the top of Gmail.
  • Forwarding & IMAP/POP: Set your account to forward to another account, or setup your account to be accessed by other software, such as mobile apps or desktop clients.
  • Chat: enable/disable Google Chat, and customize some Chat options.
  • Web Clips: show/hide single-line advertisements at the top of your Gmail window.
  • Labs: Enable/disable new features that Google is providing on an as-is basis that may or may not stay around.  See below for some recommendations.
  • Offline: Install & configure Gmail offline. This is an unsupported beta add-on for Google Chrome that allows you to read old mail if you are disconnected from the internet.
  • Themes: Options to customize the background and other parts of your Gmail screen.
  • Other tabs: You may see additional tabs in this Settings window if you have enabled some Google Labs features.