Species is the word biologists use to describe organisms which are closely similar in appearance and which can breed and produce fertile offspring. All humans are the same species even though there's lots of variation in your colour, size, hair type and so on. I am a King Penguin and I can mate successfully with any female King Penguin - if she likes me. We Kings are all the same species. My mate will lay fertile eggs (usually two) which should hatch into brown fluffy chicks. But I can't mate and have chicks with other types of penguin because they are not the same species as me. What about a crab and a toucan? Well they are definitely not the same species! Pretty obvious really. For a start, crabs have 8 legs and toucans have two. And crabs don't fly... How many species can you think of? And how many species are there on the planet? Scientists estimate that there are at least 5 million but there are probably many tens of millions more which we still don't know about, mostly living in the oceans.