This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
This is referred to as "dependent dropdowns" or "dependent selects". It has been discussed recently in this forum in this thread. In addition, you will find a lot of information by "Googling" this topic.
After reading the other thread, let us know if you have more questions.