From the article if you couldn't read it:
a database containing private user information has been stolen.

That information includes user names, hashed and salted passwords, game purchases, email addresses, billing addresses and encrypted credit card information, according to an email sent by Valve managing director Gabe Newell to Gamasutra.

According to Newell, the company does not currently have any evidence of credit card misuse at press time, though warns that Steam users should nonetheless closely monitor their credit card activity.