This isn't just a simple light resistor. The lightBUG is a luminosity / lux / light sensor based around the TSL2561T, enabling you to measure infrared, full spectrum or human visible light. And to help store any data collected, the BUG also has 128Kbit (16 KB) of onboard EEPROM. Our fiends at Adafruit have a great article on what can be done with this light sensor.
This is a quarter size stickyBUG and occupies one quarter of a stickySHIELD. This means you can have four quarter size bugs per shield or you can have one half size BUG and two quarter size BUGs per shield. They can be stacked on other stickySHIELDs if required. Only one lightBUG can be used for any given design.