Before moving on to individual big cats, lets first establish what big cats are - i mean how are they differentiated from small cats...there is actually some argument on this issue in certain circles!
Generally speaking big cats are 1) Wild and 2) Considerably bigger than the small domestic or wild cats (a Siberian tiger for instance may be up to 100 times heavier than our 8 pound domestic feline).
Another commonly employed classifying method is the ability to roar. Big cats have an elongated larynx and elastic hyoid bone that enables them to roar (and prevents them from purring). Small cats are the opposite. Their hyoid bone is ossified, thus they are unable to roar, and may only purr.
Other interesting difference is the shape of pupils. All big cats have round pupils and small cats, apart from lynx, have slit-like pupils (exaggerated in bright light). Also big cats usually consume their meals lying down, apart from snow leopard, whereas house cats - as we all know- eat in a sitting position!
Many of the earlier classifications used to include only four species: Tiger, Lion, Jaguar and Leopard into big cats. However, today big cats are expanded to include also Cougar, Snow Leopard, Clouded Leopard and Cheetah. Despite significant differences in size in all the big and small cats, all felines are remarkably similar in their basic body make-up and sensory perceptions.