From Live Nation:

LOS ANGELES (February 13, 2013)— Live Nation Global Touring has confirmed that the remainder of the Lady Gaga Born This Way Ball performances have been canceled.

After additional tests this morning to review the severity of the issue, it has been determined that Lady Gaga has a labral tear of the right hip. She will need surgery to repair the problem, followed by strict down time to recover. This unfortunately, will force her to cancel the tour so she can heal.  
Refunds for the canceled performances will be available at point of purchase starting Feb 14, 2013.