Duplo bricks are eight times the size in volume, twice the length, height and width of traditional Lego bricks, and are easier to handle for younger children. Despite their size, they are still compatible with traditional Lego brick.
Duplos are great to introduce kids to the concept of Lego bricks and to get them to think about building their own creations. However you would freak out if your kid would grow up and not want to start playing with Legos and building more advanced/custom stuff.
Unfortunately, that's exactly what's going on in the Rails community right now. We created a generation of Duplo developers.