Auto populate subcategory select box based on category choosen

How do I populate 2nd select box that holds subcategories based on previous select box which contains (categories). I need a simple example. Example: Select box 1 contains: Select your OS:(categories) Linux Windows Lets say user chooses Linux Select Box 2 (autopopulates with subcategories for Linux loaded from database: ) program 1 for Linux program 2 for Linux Select box 1 contains categories and select box two will populate with subcategories based on the category choosen loaded from a MySQL database. The data is stored in a single database called database1 using one table that contains category and subcategory. I know I have to use javascript but I need a very simple example on how to do this.