This is a very interesting, well-written article on the subject, not too long and verbose. I'm certainly very green when it comes to understanding transformers and electronics in general. Boy, do I miss my Dad now that I've become interested in audio? He was an electrical engineer and designed really big installations and transformers the size of large houses, so I'm sure he could have helped me out with the baby ones we use!
In this article, author Pete Millett explores the different types of power transformers, and why you might choose to use each type in audio equipment. The article addresses issues with low-frequency (50–60Hz) line transformers, but many of the same issues apply to audio-frequency transformers as well.