What is an application for Mac OS X that let's others view your desktop over the internet?

I would like to teach a friend how to use Adobe Photoshop, but we don’t live near each other. I would like to find an application that would let her see me work with the Photoshop from her own computer. I thought it would be good for her to see me using an application and hear me explaining it. This would be a great alternative to being there in person.