Smart grid systems are complex and huge power networks including many digital components and assets. These systems provide opportunities to operators to collect data remotely from customers and manage their network reliably and effectively. Therefore, the operators can track and analyze historical and immediate power consumption data, and take necessary actions to manage changes or meet the requirements of the consumers. Beside smart grid systems bring many advantages compared to traditional power systems, cyber security is one of the main challenging issues for these systems. In this paper, vulnerabilities and threats for smart grids were reviewed, categorized and evaluated for six components of smart grid systems, for the first time. Then cyber security considerations on smart grid systems were examined and discussed, and finally some applicable measures to minimize currently available vulnerabilities and threats were presented.