Bluefruit LE (Bluetooth Smart, Bluetooth Low Energy, Bluetooth 4.0) nRF8001 Breakout allows you to establish an easy to use wireless link between your Arduino and any compatible iOS or Android (4.3+) device. It works by simulating a UART device beneath the surface, sending ASCII data back and forth between the devices, letting you decide what data to send and what to do with it on either end of the connection.

The nRF8001 is nice because it handles all the BLE radio and low level work, and does it all over SPI which makes it easy to use with any kind of microcontroller. All pins you need are broken out on the bottom of the PCB and all are 5V compliant so you can use with 3V or 5V micros!
Starting from the left:
SCK - this is the SPI data clock pin, connect to your SPI master clock out
MISO - this the SPI data out pin, data is sent from the module on this pin. Data level is 3V but that is fine for 5V microcontrollers.
MOSI- this is the SPI data in pin, data is sent to the module on this pin.
REQ- this is basically what the nRF8001 considers the 'SPI Chip Select' pin, its an input.
RDY (ready) - this is the data-ready pin, an interrupt output from the breakout to themicrocontroller letting it know that data is ready to read
ACT (active) - this is an output from the module, it lets the host know when the nRF8001 is busy.
RST (reset) - this is the reset pin input.
3Vo - this is the output from the onboard 3.3V regulator, you can grab up to 100mA from this pin.
GND - common ground for data and power.
VIN - 3-5 VDC input to power the brea.