Tesla coil is an invention of Nikola Tesla; this instrument is actually a transformer. Transformers are usually step-up or step-down types which increase or decrease the input current respectively.
The Tesla Coil is an air-core transformer with a high frequency. It is a step up transformer which increases the input of 120 volts AC to around 100,000 volts.
The discharge is usually in the form of electrical arc. These incredible coils have in the past were known to light up florescent lights which were almost 50 feet away, wirelessly. The other fact is that there is a direct conversion of electricity to light which does not need the electrodes, unlike the normal bulbs. What it means is that, the florescent lights which have already burnt out can also produce light.