Username: This is the name entered to validate login along with the password, often called a 'log-in ID' in order to avoid confusion

For example:
                                   Login
       Username: __________
       Password:  __________

Nickname: This is the name other members in the forum will see you as, for example, my nickname is 5p45T1kM0nk3 - it is an ID different to your login name so that others do not learn your username, but can still recognise who you are

The Difference: Username is used to log in to your account, it is the name you need to remember, and nickname is the name everyone else sees when they read your activities, posts or account, and is the name other users need to remember