Constitutional Law is the study of foundational laws that govern the creation and execution of other laws by a government. Not all nations have constitutions; of those that do, not all have written constitutions (compare, for example, the written Constitution of the USA with the largely traditional but still very forceful Constitution of the United Kingdom, which arises from multiple sources including Magna Carta, the common law, and other customary sources).
A government is a body that creates rules for the orderly conduct of a society, and that is acknowledged by all (most?) as being legitimately able to use force to ensure compliance. A Constitution binds a government, limiting the contexts in which rules may be created and force may be applied. It is important to understand that no outside body enforces a Constitution.