Where they go: /Applications/
What to use these for:
MacHD.app - If you don't want to use Categories to make and HDD than use this to make your spingboard (home page) look more like a mac.
Trash/Trash_reflect.app - These are simply dummy apps designed to look like Leopard's trash bin. They aren't functional at all but they look great on the dock.
stackbg.app - If you use a modded version of Stacks that doesn't generate an icon for the stack on your dock then replace the current 4th item with this and put all your folders in to the stack.