Q:

What are LDUs and how do they relate to LEGO bricks?

What are LDUs? I see technical people talking about plans and designs in terms of LDUs. I understand it's a measurement unit.
What does it mean and how do LEGO bricks relate to them? Why are they needed?

A:

LDU stands for LDraw Unit - the basic unit of measurement in LDraw, the open standard for LEGO CAD programs, and map to the "Fine" grid setting in applications such as MLCad.
Common measurements include:
1 brick width/depth = 20 LDU
1 brick height      = 24 LDU
1 plate height      =  8 LDU
1 stud diameter     = 12 LDU
1 stud height       =  4 LDU    

A 1x1 Brick as 3 Plates in LDUs (via LDraw)
The typical LU (LEGO Unit) measurements are coarser at:
Brick height = 6
Plate height = 2
Stud pitch   = 5

Based on the physical dimensions of a LEGO brick, this means that 1LDU == 0.4mm, based on a stud height (1LU) of 1.6mm.

