We produce several different Raspberry Pi boards designed to cover different electronic needs.
RTC Pi:  This board contains a real-time clock with a backup battery and is used to keep track of the time when your Raspberry Pi is switched off.  The battery will keep the internal clock running for several months.  If you have a project which needs to keep track of the date and time then this board will be useful for you.
IO Pi:  The IO Pi is a digital expansion board which allows you to control 32 separate ports.  Each port can be set as an input or output so you can use it to switch other devices on or off or it can detect if there is a voltage on the pin.  All of the pins are digital which means they are either on or off (5V or 0V).
ADC Pi:  This is an analogue to digital converter board which means it measures an analogue voltage between 0 and 5 volts on the input pin and converts it into a digital number which can be used by the Raspberry Pi.  The ADC Pi contains 8 analogue inputs and is useful for measuring positive voltages.  If you want to measure a voltage greater than 5 volts then you should use a voltage divider circuit to divide the voltage down to a level between 0 and 5 volts.
ADC Differential Pi:  The ADC Differential Pi is similar to the ADC Pi in that it is an analogue to digital converter used to measure analogue voltages.  The difference between the two boards is while the ADC-Pi can measure a voltage between 0 and 5 volts the ADC Differential Pi can measure a differential voltage between -2.048V and +2.048V.  This makes the Delta-Sigma Pi useful for measuring the potential difference between two points in an electronic circuit where the potential difference may be positive or negative.