Iguazú Falls sit on the border between Argentina and Brazil. 275 individual cascades spread across a 2.7-kilometre horseshoe, dropping between 60 and 82 metres into the Iguazú River gorge below. Eleanor Roosevelt reportedly said "poor Niagara" when she first saw them. Taller than Niagara, wider than Victoria, less famous than either — which is part of the appeal.
The Argentinian side has ~80% of the falls and the boardwalk infrastructure to walk among them. Three circuits do the work: the Upper Circuit (panorama across the top of the falls), the Lower Circuit (boardwalks descending to pools and plunge points), and the Jungle Train which runs 6 km to the Devil's Throat — the 80-metre-high pour-off where half the river's volume disappears in a cloud of permanent mist. You end up walking on a steel boardwalk over the gorge until you're ten metres from the edge.
The Brazilian side (a separate park, separate country, separate ticket) gives you the panoramic view looking back at the Argentinian cascades — beautiful for photos, but a much shorter visit (2–3 hours vs a full day on the Argentine side). This site covers the Argentine park only. Booking via the APN's Spanish-only system, with dynamic peak-season capacity caps, is the reason travellers fall back on Civitatis or GetYourGuide. We do the Spanish, you get the tickets, in English.