A federation divides sovereignty by subject: some matters belong to the national government, others to states or provinces, and neither level can take the other's powers, because the division is entrenched in the constitution. That distinguishes federalism from devolution, where a central legislature delegates — and could reclaim — regional powers.
Federal systems generate a distinctive body of law: which level a given statute really belongs to, what happens when both may act, and which prevails on conflict. The common law family contains every variant — the US and Australia leave the residue of power to the states, Canada gives it to the centre, India runs three lists, and the UK and New Zealand are not federal at all.
Federalism litigation under ss. 91–92 was for a century the main business of Canadian constitutional law, and remains constant — carbon pricing, securities regulation and firearms have all reached the Supreme Court as division-of-powers cases.