The show is about a young boy (named "Caillou") who plays with his younger sister and his friends. He learns about the world around him from his mom and dad. I think my sons could watch the episodes in a loop for hours at a time. Not that I recommend using a TV show as a babysitter, but Caillou does seem to teach good lessons to preschoolers.
My younger son doesn't have a huge vocabulary right now. He says, "Mama", "Daddy", "no", "ball", "duck" and some other words (and he picks up new words every week). I think he has a typical vocabulary for a kid his age. He doens't say "yes" yet (we've settled for nodding for now), but he does says "Caillou" (it doesn't sound exactly like how the rest of us say "Caillou", but we know what he means).
Back in January, my wife found a bagel shop in Washington DC called "Bethesda Bagels" which features a logo that bears an uncanny resemblance to the Caillou character.
The bagel shop also had a large doughnut that I was compelled to buy and devour. It was good!
