What is Voki?
The best way is to demonstrate it. I used the text to voice option to copy the text which tells you all about it:
Voki is intended for non-commercial use and the website is designed with educators in mind, including a teacher's corner and ready made lesson plans for using Voki in the classroom.
Creating a talking avatar is very easy and there are a few choices for appearances and how to add voice. I only tried the text to voice option and found that the voices were good but the amount of text you could select to be limited. I am not sure how much you could record if using your own voice. It does look like an interesting way for students to add interactivity to their projects and emails.
I did find that the site responded slow at times which could be a problem if several students are logged in to use it. I can't be sure that this wasn't due to my own computer so you may want to test it yourself to see if you run into any problems.