TeamViewer is an application that allows users remote access to computers over the Internet. It is often used by support personnel to provide technical support for remote users.
As of this writing, the TeamViewer website is back online (it was down per the Inquisitor article) and the last blog entry on their Blog is from 26 May (they haven’t mentioned anything about being breached in their blog). There was a tweet in the company’s Twitter feed that “We are currently experiencing issues in parts of our network. We apologize for any inconvenience caused” and a number of Reddit users are reporting signs of compromise and attributing it to the company.
If you use TeamViewer to support remote users or if you are a remote user who has it installed for support, it may not be a bad idea to remove it (at least temporarily) until all of this is worked out.