Can you tell me about google talk?

Hello friends i use to chat sometimes in yahoo and in my hot mail account among them i like a windows live messenger called paltalk scene its a good one. I think you have try it before if you not than you should cheque it.Its really a good thing.They will give you the opportunity to chart in group,you can also open a group in it and enjoy it with your group members.I have heard the name of google talk.They tell me that its more interesting and they have a offer by using it you can call to out side of your country in phone also.I have entered Google to use Google talk.Primarily i have open a g mail account but i cant go to the next step of Google talk.
Have you use Google talk before and would you help me about this matter?