The full story of Internet governance is complicated, political and still being actively disputed. This text is meant to provide enough details to help you understand how certain aspects of the system affect particular methods of restricting access. The key point is that, in some countries, all Internet infrastructure is owned and operated by governments and large regulated telephone companies. A government that wants to block access to information can exercise direct or indirect control over points where that information is produced, or where it enters or exits the country. Governments have extensive legal authority to spy on citizens, and many also go behind what the law allows, using extra-legal methods to monitor or restrict Internet use.
The Internet was developed by U.S. government-sponsored research during the 1970s. It gradually spread to academic use, then business and public use. Today, there is a global community of people working to maintain the standards and agreements that attempt to achieve worldwide open connectivity and interoperability.
However, governments are not compelled to implement Internet infrastructure in accordance with these goals or related recommendations about Internet architecture. They can, and some do, design their national telecommunications systems to have single "choke points", places where they can control their whole country's access to specific sites and services, and in some cases prevent access to their section of the Internet from outside. Other governments have passed laws or adopted informal controls to regulate the behavior of private Internet service providers, sometimes compelling them to participate in surveillance or blocking or removing access to particular materials.
Some of the Internet's facilities and coordinating functions are managed by governments or by corporations under government charter. There is no international Internet governance that operates entirely independent from national governments. Governments treat the ability to control Internet and telecommunications infrastructure as matters of national sovereignty, and many have asserted the right to forbid or block access to certain kinds of content and services deemed offensive or dangerous.
It is important to understand Internet governance in order to relate the sources of Internet censorship to the possible threats. A national government might not only block access to content, but might monitor what information people in its country access, and might penalize users for Internet-related activities that the government deems unacceptable. Governments may both define what to block and may carry out the blocking, or may create legislation, regulations, or extra-legal forces to compel the staff of nominally independent companies to carry out blocking and surveillance. Therefore, depending on a user's situation, attempting to circumvent online censorship can have damaging real-world consequences. When user safety is involved, understanding how Internet is (and is not) controlled is critical.