Yep, this is a good explanation
It retargets every 2 weeks or so for bitcoin. Depending on the hashrate during the change, it will stick to it and after another 2 weeks change again depending on how many are mining then. :)
Sorry to hijack the thread, OP may give out a more detailed explanation if he wants to. :)
This is how the difficulty is calculated,
The highest possible target (difficulty 1) is defined as 0x1d00ffff, which gives us a hex target of
0x00ffff * 2**(8*(0x1d - 3)) = 0x00000000FFFF0000000000000000000000000000000000000000000000000000
Pooled mining often uses non-truncated targets, which puts "pool difficulty 1" at
0x00000000FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
So the difficulty at 0x1b0404cb is therefore:
0x00000000FFFF0000000000000000000000000000000000000000000000000000 /
0x00000000000404CB000000000000000000000000000000000000000000000000
= 16307.420938523983 (bdiff)
You can read up more about Bitcoin difficulty here as well,
https://en.bitcoin.it/wiki/DifficultyLet me know if you have any other questions (: