Mask breakdown

hi all , im learning flash as a designer because i cant find direct integration bewtween middleware and flash , so heres my question :

is there a site with a “mask” breakdown ; ie. what it is , how to use it , and some specific examples ?

my site is under construction here :
------- i need a new border (i cant get the transparency to work ) and i want to add discs for buttons and apparently if i wish to make them grow in size and contain images then i would need a mask . however i know nothing about this program except the basic “photoshop-like” functions , and the animation timelline , which is very much like "3d studio max " .

plz if you could give a break down or tutorial link , maybe with some code examples since coding and action script are very dense skillz and ive just begun .