Google Talk is pretty boring. It's an instant messaging service, but there are a ton of others out there. Gmail lets you use it to message other users as you check your mail, but I can do the same thing with my Facebook account. Voice chat was only available for PC users, and it wasn't as powerful as cross-platform Skype.
Mac and PC users will both get to enjoy video chats from within Gmail within the next couple of days. You'll have to download a client the first time you use it, but after that you can video chat with your webcam or just voice chat with a microphone. I can't wait to try it out.

Dear Marziah
I don’t know when you wrote his blog but unless you wrote it ionly a couple of days ago – which I doubt your information about Video Chat being available in Gmail Chat is incorrect! I have loaded the “required” plug in “Google Talk Plug in 1.0.10.0″ and no dice, even using Gmail Chat nor Google Talk! Please let me know if I haven’t got it right! I see many others are asking this same question so what is going on?
Have a nice day and make my day by replying to this!
Bruce
Wow, Bruce. I wrote this a while ago, but I’ve used the video chat in Gmail without any problems. I double checked, and it appears to be working fine for me. Are you using a built in video camera or a peripheral? Is the camera turned on and working ok with other software? What Web browser are you using?
We’re a bunch of volunteers and starting a new scheme in our community. Your site provided us with useful info to work on. You have performed a formidable task and our whole group will likely be grateful to you.