Why do species have different characteristics?
As we seek to label and organize the world around us we give names and classifications to different living things. We began with the Kingdoms (plants, animals, bacteria, etc.) and classified down to individual species. We refer to the science of naming animals as Taxonomy. This science was first purposed by Carolus Linnaeus in the 18th century who first came up with our hierarchy of organization.
At the very bottom of the hierarchy is the species which is defined as a group of organisms with similar characteristics that can interbreed. That means for an organism to be a different species they need to look/be built in a similar way and be able to produce offspring with each other.
A characteristic is some defining quality of an organism. For instance, hair color and height are two characteristics that many things share. The ones we do not share, feathers or tails, for instance, make us individual species and families. If an organism does not share major similar characteristics to another they would therefore be considered a different species. Some animals range by a fair amount within species (like Canis lupus familiaris) and will share some traits with other family members (grey wolf - Canis lupus) but unless they are very similar in characteristics they would be considered a different species.