Invented in 1967
and first marketed in 1969
, DUPLO is LEGO
's "under-five" product line.
DUPLO bricks are of the same proportions as LEGO bricks, but each dimension is doubled, making any given DUPLO brick eight times the size of its LEGO equivalent. These larger blocks do not pose a choking hazard to young children, and models constructed using these blocks can take quite a bit of abuse before falling apart. DUPLO and LEGO bricks are specifically designed to be compatible, and the hollow pegs of DUPLO bricks will clip into the bottom of their LEGO equivalents and vice versa.
DUPLO bricks, as with LEGO bricks, are sold both in general collections and in specific "sets". Sets are likely to contain a relatively small amount of bricks - "complex" structures like doors or automobile undercarriages are usually one piece, and DUPLO does not have anywhere near the amount of unusual and "specialty" bricks as LEGO. DUPLO bricks, as with most early-childhood toys, are almost exclusively made in primary colors. DUPLO "accessories", like signs, T-bone steaks, cans of oil, and the like, are usually one brick (two wide by one deep by two tall) with a picture of the item on one side. DUPLO does not have the "themes" that LEGO organizes their sets and models into, unless you count the extremely generic "Town", which seems like more of a default than anything. The only conclusive proof that they even exist in the same universe is the existence of DUPLO Octan figures. Or, alternately, LEGO is preparing our children for life under the yoke of a giant, multiverse-spanning evil oil corporation.
DUPLO "people" ("bulkyfigs") look somewhat more rounded and childlike than LEGO minifigs, bearing some similarity to Playmobil figures, and are not as versatile as their LEGO counterparts - bulkyfigs bend at the waist, but the legs cannot move independently, and the arms are each one piece, with no ability to rotate the wrists. However, they are modular, and the legs, arms, torsos, and heads are completely interchangeable. Earlier DUPLO figures either had immobile limbs, or lacked limbs altogether (a la Fisher-Price figures).
LEGO at one point manufactured a theme called "Fabuland", which was a bit of a missing link between LEGO and DUPLO - sets were LEGO-scale, but contained many pre-molded pieces and one-piece vehicles, and the figures were somewhat of a smaller version of the bulkyfigs. This seems somewhat unnecessary, as I believe that as is, DUPLO serves as an excellent introduction to "grown-up" LEGOs.