What is Twitter?
Twitter is a microblogger.  Remember that a blog gives people the ability to post information with others in an environment where others cannot change the post, but can comment on one.  A microblog does the same but with a limited amount of characters.  Twitter allows 140 character tweets (posts) then other Twitters can reply back (comment) on your post after they read it (in a matter of seconds, minutes, hours, or days).

How can Twitter be useful to me as an educator?
Every educator can use ideas and resources to enhance their lessons and pedagogy. Twitter has become an incredible tool to connect with fellow educators both inside and outside of your district.  Share and acquire internet links, what you are trying in your classroom,  or ask questions to others that maybe able to give you assistance or act as a sounding board. 
Nine Great Reasons Why Teachers Should Use Twitter