When naming species of anything, be it fish, reptiles, birds, etc, scientists use a system called binomial nomenclature. The first word in a scientific name is the genus, a larger, but closely related group of animals. The second word in the name is the species, such as dogs or cats. As a general rule, the genus and species name will Latin or Greek. Betta splendens is the species of fish that many people just call "Bettas". While it's the most common species in the genus Betta
and the one you see in pet stores labelled as such, there's actually about 50 other species that fall under the same genus.
Plakat is a fin type in Betta splendens. It's the first set of pictures on this post: http://www.bettafish.com/showthread.php?t=29868