Here’s a partial explanation: http://www.cnn.com/2008/POLITICS/01/02/delegate.explainer/index.html .
But it doesn’t seem to explain how Hillary got more votes in Nevada, yet Obama was awarded more delegates.
[speculation]
It's possible that the delegates are awarded based on how the candidates fared in the individual caucuses. Perhaps, similar to the electoral college, winning the popular vote nationwide doesn't guarantee a win in the election. An overwhelming win in one caucus may be effectively canceled out by the other candidate's narrow win in another one.
[/speculation]
From what I understand the voters in the Casino caucuses were worth more delegates than other Nevada caucuses. Slick Willie was complaining about that before the vote.