"Twitter is for staying in touch and keeping up with friends no matter where you are or what you’re doing. For some friends you might want instant mobile updates—for others, you can just check the web. Invite your friends to Twitter and decide how connected you want you to be.

 

 When you send in a mobile text (SMS), Twitter sends it out to your group of friends and posts it to your Twitter page. Your friends might not have phone alerts turned on so they may check your web page instead. Likewise, you receive your friends mobile updates on your phone."