They are often referred to as CdS cells (they are made of Cadmium-Sulfide), light-dependent resistors (LDR), and photoresistors.Photocells are sensors that allow you to detect light. They are small, inexpensive, low-power, easy to use and don't wear out. For that reason they often appear in toys, gadgets and appliances.
Nearly all photocells will have slightly different specifications, although they all pretty much work the same. A Photocell is basically a resistor that changes its resistive value (in ohms) depending on how much light is shining onto the squiggly face on the LDR.