What does block on Twitter do?

Answer 1

When you block someone on Twitter, they are no longer able to

1. follow you (they will be removed from your followers list if they are currently following you. If they try to follow you, it won't work, though no error message will appear).
   
2. read your tweets on their timeline.

They will not be notified that you have blocked them. A person may notice that your tweets are no longer showing up on their timeline.

Blocking has NO affect on the person's ability to:

1. Read your timeline on your profile page
2. Reply to a tweet you've made by mentioning your account with the "@" symbol in front of your account name.
   
3. Repost a tweet you have made.
4. Subscribe to your timeline via an RSS feed
5. Follow you from another Twitter account the person may own.
   

Answer 2

Block on Twitter does several things to the person being blocked. According to the Twitter help center, blocked users can't "add your Twitter account to their lists, have their @replies or mentions show in your mentions tab... follow you, see your profile picture on their profile page or in their timeline".

For more information: (See related Link)

Answer 3

If you block someone it will prevent them from viewing your profile and tweets.