hmmm....i i buy a new avr mcu,would it have bootloader?if not,then how to i put a bootloader in it?do i need to build a seperate circuit for programming with seperate bootloaders?
If you buy a new AVR uC then it won't have bootloader on it , you have load bootloaders on it using the Arduino as ISP or AVR MK-II programmer or BitBang mode or make parallel or serial programmer(Hardest to do for you) or Buy a Arduino bootoader ecquipped uC many of the vendors in India sell bootloaded ATmega chips including me.
WHat you mean specifically with it.
what is this,some another bootloader which i have to put into mcu?please help
That bootloader is based on the Human Interface Device class meaning that when you connect your computer with the chip the bootloader acts/reacts with the computer similarly a HID interfaces information and other stuff here and there.
There are Specfic bootloaders for Arduino you dnt have to care for these.
if you want to make a HID device use the USB Host shield then you can interface almost anysort of HID input device with arduino like keyboards ,mouse etc easily.