You may be surprised to learn how many bloggers post to the support forums every day who have issues centered on failing to comprehend the relationships between username accounts and associated email accounts, blogs registered under them, email addresses, cookies and log-ins.
One Unique Identifier email address per username account
When you register a WordPress.com username account the email address you provide becomes your unique identifier when you are logged into WordPress.com.
Each username account must have a different email address.
All blogs registered under the same username account are associated with the same email address.
Changing your Email Address
You have two separate email address settings at WordPress.com and you have the option of entering two different email addresses at these locations:
Blog notifications for the admin (comment moderation, Likes, new subscriptions, etc.) are sent to the email address at Settings -> General in the Dashboard.
Personal notifications (comments on your posts, subscription emails, and upgrade renewals) are sent to the email address at Users -> Personal Settings in the Dashboard.
See also > Issues Changing your E-mail Address
Username Accounts and Blogs
When you register a WordPress.com username account the matching blog URL is reserved for your future use. Reserved blogs cannot be obtained by anyone other than the person who has registered the matching username account.
WordPress.com blogs come with 3000 megabytes (~3 GBs) of media storage space for uploaded files and images. If you need more space you can buy a Space Upgrade.
There are no bandwidth limits on WordPress.com blogs, so you can publish as many posts and pages as you like.
You are not limited to registering a single blog, you can register additional blogs under the same username account and the associated email account. And, you have blog visibility options. You can either hide or make any or all of the blogs registered under a username account visible on your Dashboard.
Clicking My blogs – Show Hidden will display all your blogs registered under the same username, including the hidden ones. If you cannot see a blog at My Blogs then follow this my blogs – dashboard visibility guide.
Regardless of how many blogs you register under the same username account you can only link your username to a single “primary blog”.
Cookies and Logging In and Logging Out
The Admin  link on your blog(s) is only for your convenience and the convenience of your Editors if you have any. No one else will be able to view the Admin  link. Only you and your editors can see it and use it while logged in to WordPress.com.
As cookies keep track of commenters if someone submits a comment on your blog or submits a contact form, the next time they come to your blog, WordPress.com will recognize them and complete the details for them, unless they have cleared their cookies. No one else will be able to view those details in comment boxes or in contact forms.Commenters will not see each others details.The next person will see blank fields.
If you have more than username account then you must log out of one account before logging in under another username account. For your convenience when commenting WordPress.com provides a “change” [username accounts] link.
Separate username accounts and Blogs
If you plan to have blogs that you do not want to be associated then register separate username accounts first and then register the blogs separately under them. The email addresses for each username account must be different.
If you have a single username account and have already registered two (or more) blogs that you do not want to be associated with each other any longer, you can register another username account using a different email address and transfer one (or more) of the blogs to the new username account.
Multiple Username Accounts and Blogs
Blogs aren’t owned by institutions, businesses, organizations or groups. The sole owner of a blog is the person with the username account who registered the blog and who is the Admin of the blog.
WordPress.com and Gravatar Together Forever
As a WordPress.com username account is also a Gravatar account you can have only a single Gravatar image for each username account. But note that your Blavatar image does not have to be the same image as your Gravatar.
You don’t have to be a WordPress user or blogger to register Gravatar but it’s free and easy to do so.You can add your Verified Services links like those for Facebook, Twitter and LinkedIn, etc. and your Personal Links such as links to your blogs on your Gravatar profile page.The data you enter at > Users > My Profile in the “About you” section in your blog will also appear on your Gravatar profile page.
If you select a theme that’s designed to display an author profile then your Gravatar and that brief author profile data you you enter at > Users > My Profile in the “About you” section in your blog will appear at the end of your posts. Advice: Do not create a lengthy biographical entry for your author profile. The lengthy version belongs on your About page. Instead create a Twitter length author profile so it can do double duty and is easily recognizable to blog followers and Twitter followers.
Purchasing Upgrades for a Blog
You must be logged in under the same username account that registered the blog, who is the Admin of the blog to buy upgrades for any blog. The My Upgrades page is where you manage your upgrades. Upgrades can be transferred between blogs, if and only if, the blogs are registered under the same username account.
Changing your Username or Display Name
You can change both your WordPress.com account username (the name you use to login) and your display name (the name that is seen on your posts and comments).
You can change your “display name” or “nickname” that will appear on all blogs registered under the same username account, but do note there is no going back if you change a username, the discarded username name will not be available for you or anyone else to use.
Read about Deleting Accounts, Blogs and Content.
Recovering An Account
If you can’t login to your blog because you cannot remember the password visit the lost password page.
If you can’t login to your blog because you cannot remember the email address and/or the username, visit the lost password page, scroll down to the “Need More Help” link and complete the form for account recovery.