Forms of Computer-mediaded Communication

Text Chat: Like the book says, it involves a synchronous form of communication.  Students use this tool in their regular lives to communicate from friends who live in the same city or with people from other countries.  They just have to sign up to an instant messaging program and add your friends to it.  You can have conversations with one person or with many at the same time.  
In Computer-Assisted Language Teaching, this is a great tool since students may use it for instant-communication to share ideas about the class with their classmates and the teacher in a fast and easy way.  It is easy for the teacher to give feedback since students will see it right away.

Video Conferencing: Video conferences are a great tool too, they are very similar to text chats, the difference is that with video conferences, you can see the person and listen to his/her voice.  They are more commonly used in the business market, it is cheaper than traveling to have a meeting and it is as instant as being personally. 

Language teachers can use them to communicate with their students, sometimes it is not possible to be in a classroom, this way, students can take the lesson wherever they are and it is also easy and cheaper than moving from one place to another for teachers and students.  With video conferences, students can participate and they usually also have a chat to share information.