RAM installation chart

Each memory DIMM has one set of control circuits ("Hey can I write this to you?", "Sure where do you want it?", "What options have I got?", "Well I have {insert number of ranks} big blocks of memory for you to right to", "Ok I'll write it to block {x}, memory chip {y}, cell {z}", Ok, done, seeya!") - basically a Dual Rank (DR) DIMM has one control circuit and two ranks of memory chips, a Quad Rank (QR) DIMM has four and an 8R has....yep, eight. The x4 or x8 essentially means how many big physical memory chips are on the DIMM. That's a simplistitic view but it's good enough I reckon.

As for what your motherboard will take, well it looks to be a quite simple setup - four DIMM slots per CPU, one channel per slot (biggest motherboards allow for 3 and 4 slots per channel) and each slot/channel can take any of those options and still maintain the maximum speed (2133 in your case) - multi-DIMM-per-channel motherboards allow for more memory but they often have to slow the memory down the more DIMMs there are per channel.

Hopefully that's clear enough.