Wt is Basic Law????

The Basic Law of the HKSAR was enacted in April 1990 by the National People Congress. It is then the constitutional document of the HKSAR, prescribing the system and the political structure to be practised in future.
- Protect the right of private ownership and freedoms of its residents;
- Socialist system and policies shall not be practised in the HKSAR
- Article 5 provides "The socialist system and policies shall not be practised in the HKSAR and the previous capitalist system and way of life shall remain unchanged for 50 years.
- Article 19 states "the HKSAR shall be vested with independent judical power, including that of final adjudication"
- A high degree of autonomy for HKSAR (in executive, legislative and independent judicial power.
- It is important for judges to be independent from the executive and legislative arms of the government.

You have asked two questions, one is "What is basic law" (small letter b), and the other is "What is Basic Law (of Hongkong)" (big letter b). They are different things.

1) First question, what is basic law in general.
Just in case when it is no legitmate body to enforce consitution (e.g. West Germany), and in order to maintain public order for democratic society, we need to suspend the consitution and rely on another set of "law" that governs the limits and responsiblity of the government, inlcuding the limit of law making body. That is basic law. This is exceptional occasion for a government to admit its illegitmacy. Mostly, a government will claim complete or at least partial legitmacy (e.g. Taiwan) even if it is elected by only part of the nation, or even not popularly elected at all (China).

2) Second question, what is Basic Law of Hongkong.
Basic Law of Hongkong is not truly basic law as described above. Instead, Basic Law of Hongkong is a set of law working under the PRC consitution article 31. The function is law is to replace the enactment most of national laws, which specified the limits and responsiblity of the lower part of government. That means Hongkong will have a special set of rule that specified the limits and responsiblity, without regarding those inapplicable national laws.