Gmail has a built-in Google Chat gadget that lets you send and receive instant messages via Google Hangouts directly from your email application. You can also chat with people outside your domain using the standalone Google Hangouts client

Send instant messages
 Send instant messages Quick steps on using Google Hangouts to send instant messages and use voice and video chat
 Hangouts in Gmail video 0:59 Discover voice and video chat right from inside Gmail
Google Hangouts/Chat Help Center
Find out more about using IM, voice, and video chat with the Google Hangouts gadget in Gmail.
Download the Google Hangouts client
Download the standalone Google Hangouts client to chat via IM, voice, or video