HTML 'iframe' as masking

Hi! all dis is dheeraj, m new to dis forum. and here is my very first thread:
I m facing a problem to align a page inside an ‘iframe’ (arranging x, y inside frame window) to show a particular portion of any web page through ‘iframe’.

More elaborately said can we use ‘iframe’ as somethin similar to masking in photoshop.

Any help would highly appreciable!

thanks & regards,
dj