Some countries stand by a single document constitution, some a selection of uncodified jus commune (‘common law’ or ‘law of the land’) but almost all have some body of law outlining the powers and duties of the government alongside the rights and responsibilities of its citizens. This is constitutional law.
Since Ancient Rome and Greece, democracies have operated on a trias politica - or ‘separation of powers’ - principle splitting a government’s power equally into three branches in order to guard against an authoritarian system wherein a sole individual or oligarchy may hold all these powers, proving a serious threat if corrupted. As constitutional law derives methods and parameters for administration from a constitution, codified or not, it must deal significantly with relations between these three branches of government, namely the legislative, judiciary and executive branches. Their relation might seem simple, with the first drawing up laws, the second interpreting them and the final implementing them out, but there are many subtle and complex details in communication and cooperation, not to mention these rules’ applications to civil law, which require years of dedicated study from constitutional lawyers.