how does this differential ddr receiver work?
That is just a very basic CMOS differential amplifier.  The symbol at the bottom is a current source.  The two transistors in the middle are the ones that actually amplify the signal, and the two at the top form a current mirror load.  The current mirror not only serves to increase the gain, it is also easier to build on a CMOS chip than a resistive load.  The current source at the bottom would also be implemented as a transistor in a current mirror.  

When Vin rises above Vref, the transistor on the lower right side tries to turn on and pass more current.  The current source resists this and the voltage across it increases, turning off the transistor on the lower left, shunting current away from the left side and towards the right side.  This results in a bigger voltage drop across the upper transistor on the right side, driving Vout towards ground.  

When Vin falls below Vref, the transistor on the lower right side tries to turn off and pass less current.  The current source resists this and the voltage across it decreases, turning on the transistor on the lower left, shunting current away from the right side and towards the left side.  This results in a smaller voltage drop across the upper transistor on the right side, driving Vout towards Vdd.  

The current source provides a high source impedance that increases the change in Vout with respect to the drain current of the input transistor.  

For Vih and Vil, these are just specifications.  The point is that you need to guarantee a 1 at the input is at least Vih and a zero is at most Vil, otherwise it is not guaranteed to be interpreted correctly by the logic.  Generally there will only be one actual threshold, but it could be shifted a little bit due to PVT variations.  There are some cases where you will actually have multiple thresholds, mainly when you have some sort of hysteresis.