DUPLO, introduced in 1975, produced larger LEGO bricks so that toddlers wouldn't die horrible deaths by swallowing them. They didn't market it using those words, though. It was primarily marketed towards children aged 1-5 and whoever wanted to get those darn promotional bricks that were made DUPLO for some reason.
DUPLO bricks are two times as large as normal LEGO bricks. Elements' designs are often much simpler than System elements and have much less use overall. After all, though LEGO is practically an art form, DUPLO is just a toy, and that's what it will always remain.
DUPLO's subthemes take place in a world where everyone is chronically obese and nobody uses any form of glass to seal cockpits, vehicle interiors, and windows. Buildings will also frequently forego roofs.