Help me understand bottom brackets and cranks I have a Norco Threshold cyclocross bike (http://www.norco.com/bikes/road/cyclocross/threshold-carbon/threshold-sl-ultegra/)
. It has a PF30 bottom bracket and an FSA Energy Cross PF30 46/36T crank.  

The BB is making a lot of clicking and creaking noises, and if this cannot be fixed, I am thinking about replacing the bottom bracket with a Praxis Conversion BB (http://www.praxiscycles.com/conversion-bb/). That BB is designed to replace a PF30 BB. It is supposed to work with "Shimano™ Hollowtech II steel spindle road cranks only".  Praxis also offers variants of the same Conversion BB for Shimano Mountain, SRAM road/mountain, Campagnolo. I assume I can ignore the Campagnolo variant.

I am trying to figure out if it will work with my existing FSA Energy Cross crank. I am unable to tell what the difference is between various types of cranks. A lot of websites talk about "24mm standard" but I have no idea what that means. Is it the same as Shimano Hollowtech 2 ?  What characteristics must be the same for two cranks to be compatible ? Diameter, Length, something else ?

This is the crank: http://www.fullspeedahead.com/products/cranksets-cyclocross/energy-cyclocross-crankset-4636-bb30/  On the page, it says "BB30", but it also has a "PF30" logo. Does that mean it works with both types of bottom brackets ?
24mm refers to the diameter of your spindle (the rod connecting the two cranks)

You can look for more details on this chart:
http://www.competitivecyclist.com/images/sizing/FSA___RD_CRANKSET_STANDARD_18x24_20140306_reduced_size.jpg

Hope this works for you :-)