how usb devices work with Arduino

i think iam not very clear about this and was looking to use usb devices but i must first clear my concept on how usb works with arduino. so if i have to connect a usb device say a usb blue tooth device or usb modem , these devices would need drivers made specially for Arduino ? what about usb host sheild ,would i need that too ? i would be greateful if someone can explain all this .

thanks

Yes, you will need a USB Host shield and a library to handle it, as well as the knowledge or libraries to handle anything you plug into it. There is a shield, and a libray to handle it, but there are libraries to handle only a few device types.

For the Uno/ Mega and there clones yes you need a USB shield as these devices are slaves not hosts. The Yun does provide a USB host port. A Yun looks to cost less than an Uno + a USB host shield.

Mark