A phototransistor is an electronic element that relies on light as the gate control mechanism and current regulator.  Most phototransistors are made in the form of a bipolar transistor, meaning that the base-collector-emitter structure is used.  The main difference is that the base semiconducting material is designed so that it is sensitive to a light source.  As photons enter into the base structure, they are converted into a current flow that acts as the BJT base current that acts to enable the transistor.

The phototransistor is housed in a transparent casing to allow for easy light passage.  Often, they have casings that help to enhance and focus light entry to the critical and sensitive components of the transistor.  When the base current is formed from light entry, this allows a large amount of current to pass from the emitter to collector.

Since light acts as a switch in the case of a bipolar phototransistor, these devices are used in many electrical circuits that have important light sensitivities.  This could include fire alarms and computer equipment like CD players or infrared devices.