hard drive storage ?

I know people normally use external hard drives as a back up, but can they be used as if it was you internal hard drive.

Can you clarify what you mean by this? You can actually boot off an external, should you so desire.

well say if i was to copy my itunes library to an external hard drive, and delete it from my macbook, would i be able to access and use it instantly once i connect it to the external hard drive, or would i have to download or copy it back to my labtop before i could use it again.

You can access your iTunes library on any device you got mounted. So, if you have it on an external, it'll work. But you (obviously) have to have it plugged in. My wife's iTunes library is on a server.

How to move an iTunes library to an external drive | MP3 Insider - CNET Reviews

Thanks