Sensing Ports

The Sensing Ports are the ones exposed on the relative coordinates of the board. There you can attach sensors and actuators.
The North has the Analog pins while the South has the Digital ones.

The NW port has the Analog pins A4 and A5.
These pins are also SDA and SLC respectively.
SDA and SCL corresponds to the I2C protocol signal pins as "Serial Data" and "Serial Clock". The I2C protocol is very popular. There are may devices available out there like sensors, actuators, displays, etc.

The NE port has the Analog pins A0 and A1.
Analog pins are useful for attaching sensors like pots, LDRs (Light Dependent Resistor), piezoelectric transducers, etc.

The SW port has the Digital pins D2 and D4.
Pin D2 is very useful as it can be connected to INT0 (Hardware Interrupt 0). Therefore, if a high priority sensing is required, it can provide the trigger for an ISR function.

The SE port has the Digital pins ~D9 and ~D10.
Both pins are capable of PWM (Pulse Wide Modulation), which is useful for dimming LEDs and many other applications that require this type of modulated signal.