Dinosaurs were simply extinct reptiles in the same category as living snakes, lizards, and turtles.
While dinosaurs are cladistically classified within Reptilia, they are not 'reptiles' in the modern vernacular sense. Non-avian dinosaurs formed a distinct clade (Dinosauria) that diverged from the lineage leading to modern reptiles. Modern lepidosaurs (lizards and snakes) are more closely related to tuatara than to dinosaurs phylogenetically. Dinosaurs had fundamentally different physiology, bone structure, metabolism, and behaviour from modern reptiles. This terminology confusion reflects evolutionary systematics vs. traditional classification; while phylogenetics groups dinosaurs with reptiles, they represent a substantially different group that shouldn't be conflated with modern squamates.