You know how human beings have different types of finger prints? Similarly fish have different types of fish scales. Every fish that we know of has different types of fish scales regardless of how big or small they may be because that is just how they are made. Generally speaking the scales are hard and when you touch them they would feel slimy and smooth at the same time. These scales protect them and help them survive in the aquatic environment.
One of the many types of fish scales are the leptoid scales which can be found on certain types of fish that are bony. These scales change as the fish matures and since fish move against the current usually their scales help them do so without any friction (known as drag in physics). Think of their scales as defense mechanisms that help them survive under water. There are various types of fish scales even where leptoid scales are concerned. There are 2 types of leptoid scales- cycloid scales and ctenoid scales. Cycloid scales can be seen and felt in those fish that have smooth outer lines/margins. Typically fishes with soft fins have these types of fish scales.
Where ctenoid scales are concerned the outer edges have tiny teeth-like structures known as ctenii and this gives these fish rough textures and even if you were to hold them you could feel these textures on their fins. They have almost no bone and feel kind of like rubber toys if you touch them.
Another of the various types of fish scales are placoid scales which are also referred to as dermal denticles. You can find these types of fish scales on almost all sharks and sharks are usually covered by them all over too. These types of fish scales make it easy for the shark to swim and drag is reduced too. This is important- being bigger sharks need these types of fish scales so that they can live in the water despite their sizes. They are also able to swim quietly and this is why they can catch prey easily.
Elasmoid scales are those types of fish scales that are quite thin. Most fishes today do not have elasmoid scales and those that did are mostly extinct today. Lobe-finned fishes are believed to be quite ancient and they are covered by scales that are known as cosmoid scales. It is thought that the fusion of placoid scales is what these types of fish scales have been derived from.
The thing about various types of fish scales is that some of them have changed over the years due to evolution and in order for the fish to be able to survive under water this is necessary. Some fishes that do not exist any longer may have perished due to their inability to survive the currents of the water such as those with elasmoid scales.
These are just examples of some of the various types of fish scales that are known to us today.