At the center of the Twitter experience is the timeline, a chronological list of updates from users you've chosen to follow. If you unfollow someone, that person's updates no longer appear automatically in your timeline; however, you can still browse to the user's profile page or add the account to a Twitter list to keep up to date with new posts.
How Unfollowing Works
Unless users protect their tweets, their Twitter profiles are visible to anyone on the Web. The act of following someone means that new tweets are automatically pulled into the timeline on a constant basis -- but unfollowing undoes this. If you unfollow a protected account, you will not be able to view the tweets sent from it, even from the profile page; in addition, the user in question will not be able to send you a direct message.
If you unfollow someone, that user does not receive a direct notification about the action you've taken. The number of followers associated with the account will drop by one, which may or may not come to the attention of the account owner. The unfollowed user may also notice that there's no longer a "Follows You" label visible when visiting your profile page on Twitter.
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