Mermaid tails vary dramatically in type, depending on the creators' budget or creativity.
Used mainly in big-budget movies, such as Pirates of the Caribbean: On Stranger Tides. These can be enormous, long and highly flexible, though like most computer generated objects, it is debated whether or not they are actually better than practical effects, since their nature tends to make them less convincing. Computer effects may also be used to airbrush practical effects for mermaids. CGI can also be used for an entire body when the merfolk are not meant to be attractive, like in the Harry Potter franchise.
Latex or Silicone TailsEditThe professional-looking kind used in shows like H2O: Just Add Water. They are typically created by casting the actors' legs in plaster, building a clay model, making a mold from that, and pouring the final tail product into it. These tend to be large, durable and highly detailed, sometimes with individually-attached scales or a zipper (being less flexible than, say, a fabric tail). It may incorporate a monofin or even a foot pedal in the base of the tail for easier swimming, especially since these tails can be heavy.
So far, the only YouTube mermaid shows that seems to use one of these are Mermaid Magic by pianopup210, Tail Flip by Siren Tide, The Fin Trials by ThatClaraGirl, The Mermaid Portal, both purchased from the Mertailor, and Mermaid Miracles, from Mernation.
Fabric TailsEditOften from sites like Magictail.de and suntailmermaid.com, but also often homemade, these tails are usually made of a stretchy, water-resistant fabric (possibly spandex) and involve a monofin for the fluke. When professionally made, they are dyed to mimic scales and flukes, and usually worn with matching bikinis that come with the tail. When homemade, the monofin is optional, or sometimes homemade as well, from foam or cardboard, flipflops and duct tape. These are more unisize and flexible than latex tails, but also more clearly show the actors' legs beneath the costume.
MacGyver TailsEditFabric tails created from whatever the show's creators can find or make, which can vary wildly, depending on the creators' sewing ability. They tend towards the creative side: towels, leggings, pillowcases, duct tape. These may include a monofin or flippers, or simply have the mermaid's feet wrapped in fabric. Shows with these kinds of tails tend to avoid swimming or frequent transformations--although strangely enough, they usually keep the Splash Effect!
Mermaids will sometimes begin with one kind of tail and then magically gain a different one. Sometimes this is an upgrade from a MacGyver Tail to a Fabric Tail (Fire Ice Just Add Water, Secret Life of a Mermaid) or between Fabric Tails as the mermaids outgrow or wear them out (The 3 Tails). This is typically explained by a magical potion, celestial event, magic spell, or not at all.