Computer security is a branch of technology known as information security as applied to computers. The objective of computer security varies and can include protection of information from theft or corruption, or the preservation of availability, as defined in the security policy.

Computer security imposes requirements on computers that are different from most system requirements because they often take the form of constraints on what computers are not supposed to do. This makes computer security particularly challenging because it is hard enough just to make computer programs do everything they are designed to do correctly. Furthermore, negative requirements are deceptively complicated to satisfy and require exhaustive testing to verify, which is impractical for most computer programs. Computer security provides a technical strategy to convert negative requirements to positive enforceable rules. For this reason, computer security is often more technical and mathematical than some computer science fields.

Typical approaches to improving computer security (in approximate order of strength) can include the following:
o Physically limit access to computers to only those who will not compromise security.
o Hardware mechanisms that impose rules on computer programs, thus avoiding depending on computer programs for computer security.
o Operating system mechanisms that impose rules on programs to avoid trusting computer programs.
o Programming strategies to make computer programs dependable and resist subversion.

Data security is the means of ensuring that data is kept safe from corruption and that access to it is suitably controlled. Thus data security helps to ensure privacy. It also helps in protecting personal data.

Application security encompasses measures taken to prevent exceptions in the security policy of an application or the underlying system (vulnerabilities) through flaws in the design, development, or deployment of the application.
Applications only control the use of resources granted to them, and not which resources are granted to them. They, in turn, determine the use of these resources by users of the application through application security.

Information security means protecting information and information systems from unauthorized access, use, disclosure, disruption, modification, or destruction. The terms information security, computer security and information assurance are frequently used interchangeably. These fields are interrelated and share the common goals of protecting the confidentiality, integrity and availability of information; however, there are some subtle differences between them. These differences lie primarily in the approach to the subject, the methodologies used, and the areas of concentration. Information security is concerned with the confidentiality, integrity and availability of data regardless of the form the data may take: electronic, print, or other forms.

Governments, military, financial institutions, hospitals, and private businesses amass a great deal of confidential information about their employees, customers, products, research, and financial status. Most of this information is now collected, processed and stored on electronic computers and transmitted across networks to other computers. Should confidential information about a businesses customers or finances or new product line fall into the hands of a competitor, such a breach of security could lead to lost business, law suits or even bankruptcy of the business. Protecting confidential information is a business requirement, and in many cases also an ethical and legal requirement. For the individual, information security has a significant effect on privacy, which is viewed very differently in different cultures.

The field of information security has grown and evolved significantly in recent years. As a career choice there are many ways of gaining entry into the field. It offers many areas for specialization including Information Systems Auditing, Business Continuity Planning and Digital Forensics Science, to name a few.

Network security consists of the provisions made in an underlying computer network infrastructure, policies adopted by the network administrator to protect the network and the network-accessible resources from unauthorized access and the effectiveness (or lack) of these measures combined together.

Source: http://en.wikipedia.org/wiki/security