What are the four big cats (genus Panthera)? What makes then different from other large cats (besides genus and size)?

The largest cats are members of the genus Panthera. One definition of big cat includes only the four species of Panthera (lions, tigers, leopards, and jaguars). Members of this genus can roar while other cats cannot. Consequently, the ability to roar is sometimes considered a distinguishing characteristic of big cats.

In addition to the genus Panthera, four other species (cheetahs, snow leopards, clouded leopards and pumas), with adult males weighing as much as 150 lb (70 kg) or slightly more, are often considered to be big cats.(W)

Well, that didn't take long.

Good job, Mozart.