An acceptable use policy describes the uses that users should (or sometimes shouldn't) make of a system. While it is usually not possible to precisely enumerate all possible acceptable uses, an effective policy describes the most important considerations users should apply.
Typically an acceptable use policy covers three major areas of concern:
Legal considerations: Usually some state and federal laws apply to the operation of computer systems (such as the federal Electronic Communications Privacy Act (ECPA) or Oregon's computer crime laws) and users should be aware of the relevant laws. More recently the ability of computer users to distribute copyrighted material without permission of the copyright holders has become a sensitive issue. Also, use of computer resources in support of illegal activities can result in actions against users and the system owners.
Institutional requirements: While these may not always apply with the force of law, specific requirements of computer use in your institution (a business, government agency, or school) usually impose other limitations. For example, the University, as a state-funded educational site, has a number of state legal requirements for state-owned computer equipment. Businesses may prohibit personal use of computer resources, requiring that they be used for only business-related tasks. You may also be subject to contractual requirements made between your institution and a software or hardware vendor relating to your use of their products.
Practical requirements: These relate to any restrictions on activities that cause problems for the system in general or other users, or unwanted administrative overhead. A general term for behaviors that result in at least some aspects of the system becoming unusable is "denial of service". This can include things like increasing system load, making regular system services unresponsive; exploiting bugs in services or the operating system to crash them; or flooding services with bogus requests to the exclusion of legitimate accesses. Users may also use system services to annoy each other, such as flooding other user's terminals or mailboxes with messages or otherwise harrassing each other.