The Species Concept

All living things can be separated into groups whose members become more and more similar, until eventually we get to a group of organisms that we call a species.
The members of a species can interbreed with each other to produce offspring that can themselves have offspring.
You can represent this idea visually in a diagram like this one that shows that the number of organisms in a group decreases as you move further down the "pipe".

The XSIQ site has more!!!
Go to the first section in "The kingdoms of living things" and the last sections of "How organisms are classified" to view it.