Web 2.0 can be defined as the online tools used in order for people to connect and communicate with one another.  Teachers should know about it because it is a good way for them to communicate both with each other and with the student's parents.  It's a good way to help teachers work together and to keep parents involved.
 
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Blog is an online journal which allows people to share their stories and ideas with others.  Every teacher should have one because it is a great way to capture the student's attention.  It's something that they're in to and can be used in the classroom to complete assignments as a class or in groups.  They can learn to work together while having fun!
First of all, blogs can be about a number of different topics.  Student driven blogs can be about anything and might even be just for fun between friends whereas a teacher-created website focuses on certain subjects being taught in the classroom, for the students.