There are two main types of electronic transformers on the market: toroidal and bobbin. These two devices cannot be used interchangeably. Bobbin transformers are wound in their construction, where as toroid coils are attached to the toroid's core. In both devices, the core can be coated or boxed (this allows the toroid or bobbin to be insulated from the coil winding). A bobbin is a pre-formed part that is rather rigid. When the bobbin is completed, it looks like a spool of string-wire.
A bobbin has many applications that are not suited for toroids. This includes working with different materials such as, glass, Teflon, and plastic. The bobbin is often used in filter circuits and circuit isolation applications. They can also be used to switch mode power supplies.
A toroid should be used with metals such as nickel, steel, and iron. Different alloy combinations can be used as well. A toroid can be found inside computer power supplies, as well as radio transmitters, and EMI circuits. Toroids are more versatile than bobbins, and as such they are more frequently used.
Knowing the difference between these two devices will ensure that you get and use the correct one.