Super Marios Bros 2 is the third highest selling Nintendo title of all time. It is also known as Super Mario USA as it was not released as a Mario title in Japan.It was releasedas Yume Kojo: Doki Doki Panic. What was released in Japan as Super Mario Bros 2 was Super Mario Bros the Lost Levels in the US.
The game was different in gameplay as you didn't stomp on emenies to eliminate them. You tossed them or tossed objects on them such as bombs, vegetables and other enemies. You could also choose between four playable characters such as Mario, Luigi, Toad and Princess Toadstool. In this gamePrincess Toadstool isn't a captive as in the other titles in the series.
The game itself is unique to the Mario series. It presents a new challenge as it offers a completely different game as compared to the rest of the Mario titles. I guess being the third highest selling NES game speaks for itself.