Public law is the set of laws governing the power of public authorities and their relationship with individuals. Public authorities can be anything from a government minister to your local council, and all of the bodies involved must act according to certain rules. If someone is unhappy with a certain decision, they can ask for a judicial review of the decision.
A judicial review is where a Court will evaluate a complaint against a ruling. They have the power to invalidate decisions and laws that have been made, in order to ensure that legal powers have been exercised fairly and suitably.
Public law exists because the government is the sole body that can take decisions on the rights of an individual, and therefore they need to ensure they keep within the law when they act. Public law deals with relationships between people and the state, whereas private law deals with relationships between individuals. However, it can often be difficult to distinguish between public law and private law, resulting in a lack of clarity until a decision is made on the matter.
Administrative/constitutional law, criminal law, procedural law and tax law are all sub-sections of public law. Mandatory rules prevail for public law cases, which means that certain legal agreements can override certain rules. These agreements include contracts, trusts and wills, amongst others.