What type of big cat is a panther actually?

It's basically a black leopard. Trouble is, you can't see the spots easily because they're black spots on black fur... They are there, though.
'The leopard cannot change his shorts'

Panthers are Genus Puma, species concolor, the common names of which include Puma, Panther, Mountain Lion and Cougar. They are all the same species, with a number of subspecies, including the Florida Panther, taxonomically known as Puma concolor coryl.

Interestingly, members of the genus Panthera are not panthers, and vice versa. Panthera includes lions, tigers and leopards. It must be noted that occasionally leopards are also called panthers, but most animals commonly refered to as "panthers" are not of this genus. (At least in the US.)

Er, yes, and also what Baloo said.