You can send SMS messages to your contacts' mobile phones using Gmail Chat. To do so from Gmail:
1. Enter your contact's name in the 'Search or invite friends' box in Chat, and select Send SMS from the box of options that appears to the right of your contact's name. Or, if you already have a Chat window open for this contact, just click Video & more, and select Send SMS.
2. In the dialog box, enter a phone number in the 'Send SMS messages to this number' field. For now, this feature works only on United States phone numbers. If you're outside the US, you can still use it, but you won't see the SMS option in Chat until you enable it manually in the Chat settings page.
3. Click Save.
4. A Chat window appears. Just type your message as you would normally. When you hit Enter, the message will be sent to the phone number you entered.
If your contact replies, the text message response will appear as a reply in Chat. These conversations are stored in your Chat history just like regular chats (but keep in mind that you can’t go off the record while communicating via SMS).
The nice thing about Gmail SMS is that each user gets a virtual phone number that facilitates replying to messages. "To write back, reply to the message as you would any other text message, and your message appears as a Chat message in your friend's Gmail account. If you don't want to receive any SMS messages from Gmail, reply with the command STOP. If you'd like to block the person who sent you the message, but still be able to get Gmail SMS messages from other contacts, reply with the command BLOCK," informs Gmail's help center.