An elongated bowl is oval-shaped — roughly 18.5 inches from the bolt holes to the tip of the bowl — vs the rounded ~16.5 inches of a round-front. Two inches doesn't sound like much, but it's the difference between "I can sit on this comfortably for ten minutes" and "I'm shifting around after two."
The 2" difference matters for two things: comfort and floor space. An elongated bowl leaves about 1" less clearance to the wall opposite the toilet when measured floor-plan-wise.
TOTO and Kohler both offer compact elongated bowls — 17.5" front-to-back rather than 18.5". You get most of the elongated comfort with the round-front floor footprint. The TOTO Aquia IV, Kohler Persuade, and American Standard Edgemere fall in this category. These are increasingly the default spec for new condo and apartment construction where unit floor plans are tight.
Replacement seats — including bidet seats, washlet seats, soft-close, and heated — are sold as either "elongated" or "round" SKUs. The two are not interchangeable. Compact elongated bowls typically fit elongated seats but may have a 1/2" overhang; verify in product specs before buying. The TOTO Washlet C5 / S550e / K300 are all elongated and round versions; double-check before ordering.
The independent decisions: bowl shape (round vs elongated vs compact elongated) and bowl height (standard 14–15" vs comfort 17–19") combine. Most modern residential specs default to "elongated + comfort height" — that's the ADA-compliant pair and the new-construction default. See our comfort-height facet for the height side of that decision.