SCADA systems are the computers that control essential, complicated, and often dangerous physical processes, many of which constitute the physical infrastructure critical to modern societies. These physical processes are powerful tools, and their misuse generally has unacceptable consequences. Preventing such abuse is the goal of SCADA security. To understand misuse and how to avoid it, we need some understanding of what a SCADA system is, and how it works under different forms of attacks. These are the primary objectives of this paper.