Dinosaurs are scientifically classified as a separate group from both modern lizards and birds, though they share evolutionary connections with both. Dinosaurs were a diverse order of reptiles that dominated terrestrial ecosystems for roughly 165 million years before their extinction 66 million years ago.
However, the relationship is more nuanced: birds are actually considered living descendants of theropod dinosaurs, a lineage of bipedal, often predatory dinosaurs. This means that in a strict evolutionary sense, birds are technically a type of dinosaur—they evolved from small theropod ancestors and retain many dinosaurian characteristics including hollow bones, three-toed feet (though modified), and egg-laying reproduction.
Modern lizards and dinosaurs both descended from a common reptilian ancestor, making them distant relatives within the larger reptile family tree. But dinosaurs and lizards diverged into separate evolutionary paths hundreds of millions of years ago. Dinosaurs developed unique skeletal features, such as an upright leg position beneath their bodies (unlike the sprawling legs of lizards), which enabled their enormous size and different physiology.
When paleontologists use the term "dinosaur," they typically refer to the extinct non-avian dinosaurs—the massive creatures like Tyrannosaurus and Triceratops that lived in prehistoric times. Modern birds represent the surviving lineage of dinosaurs that adapted, shrank, and developed feathers and flight capabilities. So in summary: dinosaurs are their own group, lizards are a separate group, and birds are actually living dinosaurs.