Your Office 365 user ID is assigned to you by your work or school. It looks like firstname.lastname@example.org, email@example.com, or firstname.lastname@example.org. Your Office 365 user ID is your email address and account that you use to sign in to Office 365 for work or school.
Your Office 365 user ID is different from your Microsoft account. Your Microsoft account is the one that you use for personal services like Xbox Live, Outlook.com, Windows 8, Windows Phone, and more. Check out this video, What is a Microsoft account?, for more details.
You can't use your Microsoft account to sign in to Office 365 for work or school, unless someone in your organization shared a specific document with you via your Microsoft account. Then you could access that document using your Microsoft account.
When you get your Office 365 user ID for the first time, you'll also get a temporary password to sign in with. When you sign in, you'll be asked to create your own password. Use it with your user ID every time you sign in to Office 365. For more information about changing your password, including how to create a strong password, see Change your password.
Office 365 admins
The following information is for Office 365 admins only.
Create new user accounts
You’ll create a new user ID, with a unique user name, for each user account that you add to your subscription. The domain part of the user ID is either the onmicrosoft.com domain that you chose when you signed up for Office 365 or another domain—for example, your organization’s custom domain—that you’ve added to Office 365.
For example, if you chose fourthcoffee.onmicrosoft.com as a domain name when you created your account, and you’re creating a user account with the user name colin, the user ID would be email@example.com.
If you want to use your custom domain for user IDs and email addresses, see:
Set up and use an alternate email address
When you sign up for Office 365, you also need to provide an alternate email address that’s not connected to Office 365. This email address is used for important notifications, such as resetting your admin password, so you must be able to access the email account if you don’t have access to Office 365. After you complete the sign-up process, you’ll get an introductory email at your alternate email address that includes your user ID and the URL of the page where you will sign in to Office 365. If you forget your user ID, or if you don’t remember the Office 365 URL, look at the introductory email for this information.
Note The alternate email address you are asked to provide during the sign-up process is used for important communications, like when resetting your admin password. Any product-related communications that you choose to receive from Office 365 are also sent to this email address unless you change your contact preferences. You can update your contact information on the Me page. For more information about changing your contact preferences, see Change your contact preferences.