The source – help.twitter.com
Twitter has begun testing the Twitter Circle feature ("Close Circle"). The option will allow users to show tweets to a limited circle of people.
With Twitter Circle, a user can choose a different audience for each published tweet. For example, send one tweet to a "Close Circle" of people, and the next tweet to all of their followers. And other users won't know about the changes you make to your "Close Circle" on Twitter.
You can't retweet content that you've posted to the "Close Circle".
No more than 150 people can be added to a user's "Close Circle". You cannot delete yourself from the circle, but you can turn off uninteresting discussions. Once in someone's circle, the user will be able to see the tweets and responses, published in it.
Twitter Circle function is still at the early development stage. So, it can be used by a limited number of users around the world.