Layer masking is a very useful feature in Photoshop that allows you to hide some areas (make them invisible) of a layer. A layer mask can be altered using the brush tool. To hide areas using a layer mask, you must paint the mask using black. To make areas fully visible, you use white. Using gray will make areas translucent (not quite visible, but not quite invisible as well).
I've posted three different videos about layer masks here. They're all introductory videos about layer masks, but there are some things in one video that you might not learn from the others, so I've decided to add them all here.
This first one is by Nathan Ridley from Graphics District. He uses Photoshop 7 in this tutorial, but you can still follow along even if you're using CS2 or CS3.
Introduction To Layer Masks In Photoshop 7 from Nathan Ridley on Vimeo.
This next one is from IceFlowStudios on YouTube. The speaker also talks a little bit about vector shapes in the end. But it's ok if you're not familiar with what vector shapes are and what you can do with them. The main point of these videos is for you to learn how to use layer masks and you don't really need knowledge in vector shapes in order to achieve that. The speaker simply wanted to show an example that uses a layer mask on a vector shape. The Photoshop version used in this video is CS3.
This last one is from TutorialStop.com. The Photoshop version used in this video is CS3.
If you want to use the images in this video, here are the links:
Be sure to read the Creative Commons license applied to both images.