Text & Voice Chat

Google Talk, of course, allows instant messaging. If you're completely unfamiliar with instant messaging, it means that you can connect with someone you know, then do a textual conversation back-and-forth with them. Other IM clients offer this as well. It's the core part of IM.
Aside from textual chat, you can do voice chat. This means you can talk through the internet to anyone on your contact list, as long as they have a computer equipped with a microphone and speakers and have a broadband connection.
This is PC-to-PC calling, not voice over the internet or VoIP. In other words, you aren't going to be using Google Talk to place calls from one telephone to another or from a computer to a telephone, although that apparently may come in the future. For now, it only works between computers.
How well does it work? When I talked with someone from Google on it on Tuesday, the sound quality was great. Gary and I couldn't connect at all, however, nor could Google connect with Gary.
Gary's fault? In the end, we changed one of his settings to use a specific output device for calls rather than the default device and got it going. And absolutely -- when we talked, the sound quality for me on my end in Google was much better than with Yahoo. On Gary's end, he felt things were about the same. He also wondered if he had too many apps open for Yahoo to work properly.
Google says the voice processing system it uses is supposed to produce especially good sound quality over rivals, plus it says there's supposed to be a lot of support to avoid conflicts with firewalls.
As said, the sound quality was great to me compared to Yahoo Messenger, when talking with Gary. But I used Yahoo Messenger's voice calling earlier this month to talk with my wife while I was traveling. I thought that sounded great then. In the end, I think the jury's out. We'll know in short order how the quality of calls is as people begin testing and comparing properly this week.
Unlike Google, Yahoo's tool lets you leave voice mail if no one answers (Google said a similar feature will be coming). I also like Yahoo's audio tuning wizard, which makes it easy to know if things are working. In fact, Yahoo's tuning tool easily solved the problem I mentioned Gary was having, while in Google, we had to hunt around to figure out the problem.
Yahoo's tool just came out of beta earlier this month, a sign of its maturity. Google's, of course, has just emerged to the world in beta. Prior to this, it was tested for about a month internally, Google said.