GMail will be allowing users to replay to emails by using the GMail Chat feature. It will allow you to instantly talk to a person about the email rather than taking the time to send an email back to them. Here is how Google says that you can use the feature:
1. Log in to your Gmail account. 2. Open the message you’d like to chat about, and make sure the sender is online and available to chat. 3. Click Reply by chat at the bottom of the message. * A new chat window will open displaying the subject of the email you’re discussing. 4. Click the X in the top-right corner of the chat window when you’re done discussing the e-mail.
Google will also group your chat history (if you have it enabled) with the email message so that you have the whole “conversation” in one spot. This sounds like something a few people will use but the great thing is that if you don’t use it you will hardly even notice it.
News Source: Googling Google