The continuous development of computer network system brings both a great experience and convenience but new security threats for users. Computer security problem generally includes network system security and data security. Specifically, it refers to the reliability of network system, confidentiality, integrity and availability of data information in the system. Network security problem exists through all the layers of the computer network, and the network security objective is to maintain the confidentiality, authenticity, integrity, dependability, availability and audit-ability of the network. This paper mainly aims to discuss about the basic concepts,implementations of security mechanisms, Policies and latest threats of various systems that are upcoming today.Computer security should be seen as a basic management task. It is an extension of the duty to protect the organization’s assets against misuse or loss. Also, the information stored and processed by computers is the most significant asset of most organizations. (Some prefer to the use the term information security to describe the process of protecting computing. It plays a major role in ensuring an organization’s ability to survive as what the law calls a going concern. Increasingly, maintaining this process will involve ensuring that the organization is complying with relevant statutory and regulatory agency requirements.) Information is inevitable in all kinds of entrepreneurial activities, and must be therefore protected as assets. Information security may be assured in various ways, including related policies, processes, procedures, organizational structures, software programs and hardware equipment able to eliminate many sources of safety jeopardizing such as espionage, computer fraud and deceit, sabotage, vandalism, fire or water. Computer Security is the protection of computing systems and the data that they store or access. How many attacks to computers on campus do you think take place everyday? l Thousands of attacks per minute bombard our campus network. l An unprotected computer can become infected or compromised within a few seconds after it is connected to the network. l A compromised computer is a hazard to everyone else, too – not just to you.