LEGO and DUPLO blocks are both made by the same company, The LEGO Group. While the two function the same as how the pieces snap together, DUPLO are designed for younger children who may not yet be nimble enough to play with LEGO. The DUPLO blocks also help to eliminate the choking hazards inherent with the tiny size of some LEGO pieces.
Basic Differences
DUPLO blocks are significantly thicker, wider and chunkier than LEGO, and are designed for preschoolers. Many DUPLO sets are basic building sets designed for imaginative construction, instead of sets designed for a specific purpose. Each DUPLO brick is eight times the size of its comparable LEGO brick, or twice as high, wide and long. The bricks from LEGO and DUPLO snap together, but the size difference is noticeable -- it takes four 2x2 LEGO squares to fill the top of one DUPLO 2x2 square 2.