iCloud (Cloud storage and cloud computing service by Apple) was hacked giving out personal and private image of lots of celebrities.
Lots of images were leaked which shows iCloud was hacked on large scale. Apple is investigating what was the cause of the hack.
Maybe accounts were hacked due to guess of login and password combination or by stealing these by installation of some malware on machines of those whose accounts are hacked.
Two way authentication can be used to make accounts more secure otherwise anyone with email and password can login to anyone's account.
No proper figures of people are known who got their images leaked by this iCloud hack.
This is why there is no privacy on internet. You should only share stuff that is safe online. Keep backup of your files in safe places. Use two way authentication for all of your accounts if possible. This is the only way to prevent un-authorized access to your accounts because you'll receive code on your phone that you'll enter to access your account.
If you want to backup your data online, encrypt it first using some password that only you know. [How to encrypt individual files?] (Apple encrypts data uploaded to iCloud but if you want even more security you can encrypt it yourself)
When snapchat was hacked back in January, data of 4.6 million users was compromized. Their phone numbers were even published online.
My advise: Don't put your sensitive data on internet. This is the only way to stay safe.
So, Stay safe. Internet is not a safe place. Take care.
Update: According to Engadget, login page of Find My iPhone was vulnerable to brute force attacks. Now apple has fixed this too.
Update: FBI has launched an investigation in order to find out who was behind this massive hack.
Update: iCloud wasn't breached.