“It is not the strongest of the species that survive, but the ones possessing firearms.”
A species is what people generally consider a "type of animal". Exact definitions differ greatly - but one thing they all have in common is that the explanations require a lot of hand waving.
One common way of defining species is "animals are of the same species if they can interbreed to give fertile offspring". However, this would rule out you and your mum being the same species. How about chihuahuas and Great Danes? I bet they can't interbreed, even if they are in love. Love can't conquer physics, man.
Some people say "If they look the same, they're probably the same species". Then they wave their arms around a lot. The problem with this is your mum looks like a grue, but obviously they aren't actually the same species. Leila on Futurama looks pretty much human, but she's an alien! Likewise, that pathetic male leech fish thing that sucks onto the side of his big fleshy mate doesn't look like his bitch at all. But they're the same species! Go figure.
DNA, that twirly ladder thing, tells each animal what to be. So, hypothetically, if you know what it says, you can tell the animal what to be too. Unfortunately, DNA is far too small to see, and even if you could see it, I bet you couldn't read it because your mum never taught you how to read. Burn!
This seems to be the best definition we have at the moment. Just take a stab in the dark. Put on a blind fold, get someone to spin you in circles, then point at two animals with a long pointed stick and say "I deem thou to be the same species and I christen ye both a quantum mechanic."