Difference between Email and Username in SalesForce

In SalesForce, when you are creating a user, you have likely noticed that there are two fields: User Name and Email that most people always enter the same thing in both.

However, do you know why there are two different fields for what seems to be the same thing?  Well, here is the answer!

In the Email field, you can specify the email address that any system notifications should go to.  This means, anything triggered by Workflow Rules, Email Templates, etc.

In the User Name field, you can specify the "user name" that users use when logging in at http://login.salesforce.com

So, why would you ever have different addresses in each?  Well, for example, if you have a user who has left the organization and you need someone else to get their notifications, you could change that (original) user's email address field to the new employee.  Or, if you already have a SalesForce logon with a said email address, but want to have another SalesForce logon, you could use the same email address for both but a different user name for each.