A toroidal coil is an inductor in electronic circuits that allows for inductions at low frequencies when comparatively large inductances are required. Toroids also specialize in containing magnetic fields within the core of the device. The ability to contain magnetic flux is a direct result of winding toroids. This is the main difference between a solenoid and a toroid.
Any inductor with a closed-loop core will most likely have a high Q factor and inductance when compared to solenoids coils that have straight cores. A straight core emerges from one end of the core, and this allows for magnetic flux to escape outside the core. This is referred too as leakage flux.
Toroids, and doughnut shaped inductors, are being used more frequently throughout electronic industries as a means to reduce leakage flux and to deal with EMI (electromagnetic interference). EMI is a serious problem, which could potentially mess with more than one device.
Winding toroids allows for total B field confinement, which eliminates EMI from the inductors core. Toroids are wound in different ways for different types of functions, but regardless of application the winding is necessary for the toroid to function properly. Contact a toroid maker for more information.