The term "handle" dates back to the 1970s and comes from Citizens Band radio (CB radio), a short-distance radio communications medium. CB radio users would identify themselves by unique nicknames, which became known as handles. When online chat became popular in the 1990s, the term "handle" transferred to the Internet and became a common way for users to identify themselves online.
While handle and username are often used synonymously, they do not always mean the same thing. For example, when you choose a username for a secure website like a bank or investment site, it is not necessarily your handle. This is because the username is intended to be private and used in combination with a password to create your login. Handles, on the other hand, are public usernames that can be used to identify people online.
The most popular web service to use handles is Twitter. In fact, Twitter usernames are often called "Twitter handles." You can reference other users in a tweet using "Mentions" or the "@reply" feature. To mention another Twitter user in your post, simply type an at symbol (@) immediately before the user's handle. A link to the user's profile will show up in the published tweet and the user will be notified that you have mentioned or replied to him or her.