How do we classify living organisms?

Traditionally we have classified living organisms into groups based on their structure and morphology (essentially how they look!). This was a system developed by a scientist called Linnaeus in the 1700's. Linnaeus came up with a hierarchical system where living things are classified into a kingdom, phylum, class, order, family, genus and species. For example, dogs are in the animal kingdom, in a phylum called Chordatata, class of tetrapods, order of Carnivora, family Canidae, genus Canis and species lupus. To refer to a species we usually just use the last two groups - genus and species. This is known as the binomial system - so dogs would be referred to as Canis lupus.