In a macro-economic system, all major sectors: agriculture, extraction of natural resources, manufacturing, construction, transport, communication and health services, are dependent on a reliable supply of electricity. Targeted attacks on power networks can lead to disruption in operations, causing significant economic and social losses. When cyber networks in power system are compromised, time-critical data can be dropped and modified, which can impede real time operations and decision making. This paper tracks the progress of research in power system cyber security over the last decade and presents a taxonomy of data attacks. Nineteen different attack models against major operation and control blocks are classified into four areas: steady state control, transient and auxiliary control, substation control and load control. For each class, a comprehensive review of mathematical attack models is presented. The goal is to provide a theoretically balanced approach to cyber attacks and their impacts on the reliable functioning of the electric grid.