What is the difference between public and protected Tweets?
When you sign up for Twitter, you have the option to keep your Tweets public (the default account setting) or to protect your Tweets.
Accounts with protected Tweets require manual approval of each and every person who may view that account's Tweets.
Who can see my Tweets?
Public Tweets (the default setting) are visible to anyone, whether or not they have a Twitter account.
Protected Tweets may only be visible to your approved Twitter followers.