The BMP180 is an ultra-low power, low voltage capable digital barometric pressure, temperature, and altitude sensor. The I2C interface allows for easy system integration with a microcontroller. The BMP180 is based on piezo-resistive technology for EMC robustness, high accuracy and linearity as well as long term stability.
This module includes an on-board voltage regulator (MIC5205 marked KB33) so you can connect the module's "VCC" pin to a 5V supply, such as an Arduino's 5V supply pin. If you want to supply the module with 3.3V then connect a 3.3V supply to the "3.3" pin on the module, which will bypass the on-board voltage regulator.
On-board pull-up resistors are connected to the I2C pins, so you won't need to worry about adding them to your I2C lines. These resistors pull-up the I2C lines to the 3.3V output of the on-board voltage regulator so you can connect the I2C pins directly to a 5V logic device such as an Arduino without having to worry about logic level shifting. They are also connected to the module's "3.3" pin for when you are supplying the module with a 3.3V supply.
IMPORTANT: The BMP180 is sensitive to moisture. Do not submerge or spray the sensor with water or solvents. See page 26 of the datasheet (link provided below) for moisture sensitivity details.