Difference between CML and ECL
THis is an ECL circuit. Notice the output is from the emitters of two BJTs:



(source: Wikimedia)

This is a CML circuit:



(source: Wikimedia)

Notice the outputs are from the collectors of two BJTs.


  explain how they are related?


The CML output is essentially the ECL output with the emitter followers taken away. 

What does this mean in applications?:


ECL outputs require pull-down resistors to bias the output buffers correctly. CML does not.
ECL outputs can be combined in wired-OR configuration; CML outputs can not.
CML outputs are 50-ohm matched. ECL outputs are low-impedance and require series termination if they need to be matched (which reduce signal swing). Typically ECL is run with matched termination at the load only.
CML's common mode level tends to be very near VCC. ECL's is about 1 V below VCC.