You may have noticed a gray or blue bar that appears at the top of your pages when logged into your WordPress.com account. This bar is called the Admin Bar and it is an easy-to-access toolbar with a few shortcuts to some central WordPress.com dashboard pages.
My Site (or My Sites) will take you to the Stats page, and main menu for your primary site. From there you can create posts, pages, manage settings, and switch to the same options for your other sites, if you have more than one.
The Reader button takes you to the WordPress.com Reader where you can see the latest posts from any sites that you follow, browse topics (tags), or peruse Discover.
New Post, Profile & Notifications
On the right-hand side of your Admin Bar are the New Post, Profile and Notification buttons.
New Post simply takes you to the New Post Screen.
The Profile link (which shows your Gravatar if you have one) varies depending on whether you are in the dashboard (with the blue bar), or viewing a website (gray Admin Bar). If you are in the dashboard the Profile link will take you straight to the My Profile page, with a menu to other account-related screens. If you have the gray Admin Bar, hovering over the Profile will give you a dropdown menu with all the options on the My Profile dashboard page, including Sign Out, Account Settings, Billing History (receipts) and Security.
Clicking on Notifications will open a dropdown that displays your latest notifications, including recent likes and comments.