Dinosaurs were Giant Reptiles.

Explanation:
Dinosaurs were a diverse group of reptiles that dominated the Earth for approximately 160 million years during the Mesozoic Era, from about 230 million years ago to 66 million years ago. They were not mammals or mollusks, but rather reptiles.

1. What are Dinosaurs?
Dinosaurs were a group of reptiles that evolved from a common ancestor and diversified into various species. They were characterized by their upright stance, with legs positioned directly beneath the body, enabling them to be efficient walkers and runners.

2. Reptiles vs. Mammals and Mollusks
Mammals are warm-blooded animals that give birth to live young and nurse them with milk. Mollusks, on the other hand, are soft-bodied invertebrates, such as snails and squids. Dinosaurs, being reptiles, differed from mammals and mollusks in various ways.

3. Characteristics of Dinosaurs
Dinosaurs had several key characteristics that set them apart as reptiles:
- They laid eggs, like other reptiles, rather than giving birth to live young.
- They had scaly skin, which helped in water retention and protection.
- They were cold-blooded, meaning their body temperature fluctuated with the environment.
- They had a diapsid skull structure, characterized by two temporal fenestrae (openings) behind the eye sockets.

4. Dinosaur Diversity
Dinosaurs exhibited a remarkable diversity in terms of size, shape, and ecological adaptations. They ranged from small, bird-like creatures to massive, long-necked herbivores and formidable carnivores. Some well-known dinosaur species include Tyrannosaurus rex, Brachiosaurus, Triceratops, and Velociraptor.

5. Extinction
Despite their dominance, dinosaurs eventually became extinct around 66 million years ago, at the end of the Cretaceous Period. The most widely accepted theory for their extinction is the impact of a large asteroid or comet, which caused a global climate change and led to the collapse of ecosystems.

In conclusion, dinosaurs were giant reptiles that thrived for millions of years before facing extinction. They were not mammals or mollusks but rather a unique group of reptiles that played a significant role in shaping Earth's history.