A species is a group of animals that all look similar and can have babies with any other member of that species. Humans are a species called Homo sapiens. We are one species because we all look very similar and can have children with each other.
But the members of a species can look very different from each other. Dogs are all one species called Canis lupus. This species includes every dog breed, from great danes to poodles and even wolves.
Cats, on the other hand, are a different species to dogs because a cat cannot have a baby with a dog.