How to handle pop up windows using selenium webdriver?

How to handle pop up windows using selenium webdriver?How to handle pop up windows using selenium webdriver

Refer Way2sms site
1) When i tried to click on register here link 
2) Pop up window comes up
3) But in pop up window don't have any buttons like OK or Cancel
how can we close the opened model pop up window?
Refer Way2sms site 
(http://site23.way2sms.com/content/index.html)

Thanks in Advance!!!!
Subbu