Google Talk is a Windows application for Voice over IP and instant messaging, offered by Google.

Google Talk features.

Interoperability: In order to provide real time extensible messaging and presence events, including offline messaging and voice mailing, Google Talks uses XMPP.

Product integration: You can chat with Google Talk in Gmail.
Google Talk does not need to be downloaded to send instant messages to Gmail users. Conversations are automatically saved in the user’s Gmail account.
Google has also integrated Google Talk with Orkut. This enables Google Talk users to interact with registered Orkut users, by sending and receiving 'scraps' within Orkut.

Voicemail and file sharing: Voicemail messages can be 10 minutes long, and they're delivered to the contact's Gmail mailbox as an attached MP3 file (11 kHz mono 24kbit/s).

Google Talk "Hacks": Google Talk also has several "hacks" or things you can use to enhance your communicational experience. They include making your words italic and bold. Also many users have found out a way to log in with multiple logins by changing the target of the shortcut.

Offline messaging: Offline messaging allows users to send messages to their contact even if they are not connected. They will receive the messages the next time they go online, even thought the user who has sent it is offline.

Mobile device compatibility: Some Google Talk-compatible device are Nokia, Sony’s mylo and BlackBerry.