Elections
The mechanism behind "the people decide" is a lot more varied, worldwide, than most people who only know their own country's system realize.
Political Parties
George Washington warned against political parties, yet disputes in his own cabinet in the 1790s helped spawn the American two-party system.
Democracy
Rule by the people sounds simple until you ask which people, and who counts the votes.
Branches of Government
A system deliberately designed so that no single branch of government can get everything it wants.
Federalism vs. Unitary Systems
Do a country's regions hold protected power of their own, or only whatever the central government chooses to lend them?
How Laws Get Made
A process with so many checkpoints that most bills introduced in Congress never make it anywhere close to a president's desk.