The act of blocking a particular user from following your tweets or adding you to any of the user's Twitter lists. You block someone by clicking on their profile and choosing "Block" on the right sidebar. The blocked user can't send you @replies or use the @mention feature. If the blocked user previously followed you, your tweets will no longer be visible in his Followers timeline. However, blocking a user doesn't prevent a spammer from viewing your tweets if your profile is public. Also, it doesn't prevent your tweets from being retweeted -- which may give other users the impression that you're somehow affiliated with the user you've blocked. Blocking a user doesn't trigger Twitter to notify her that she has been blocked. Unless a blocked user figures it out, it's possible she may never notice.