Reptiles are vertebrates that have scales on at least some part of their body, leathery or hard-shelled eggs, and share a number of other features. Snakes, lizards, turtles, crocodilians, and birds are reptiles. Like all vertebrates, reptiles have bony skeletons that support their bodies.
Scales help prevent reptiles from losing water through their skin. The leathery or hard shells on their eggs protect the young from drying out while they grow inside the egg. Most reptiles can live their entire lives on land and reproduce in dry habitats. Some types of reptiles (such as sea turtles and penguins) are adapted to living in water, but even these species come onto land to lay their eggs. All reptiles also have lungs, so even those living in water must come to the surface to breath air. Reptiles are found living in all habitats but are more common in warm, tropical places.
So, why are birds considered reptiles? Birds look very different from other reptiles, they have feathers, fly, sing complex songs, and have many other unique features.
Animal groupings are most useful when they reflect the evolutionary history of the animals in that group. So, for example, "mammals" is the name of a group of animals that are all more closely related to each other than to other kinds of animals. It's not hard to imagine that a skunk is more closely related to you than it is to a turtle, because you and a skunk both have fur (yours is mostly on your head!), you both have mothers that fed you milk, and other features. Even though you and the skunk look very different, you share features that show you share an ancestor. The "mammals" group name reflects that shared history.
Snakes, lizards, turtles, crocodilians, and birds also all share an evolutionary history. Many years of research has proven that the ancestors of birds were bird-like dinosaurs (visit the Dinobuzz page for more on this). Even though birds look very different from other living reptiles, they’re most closely related to alligators and crocodiles and clearly belong in the reptile group.