I found a helpful article by Ilene Strizver on one of my regular reads, creativepro.com that includes the diagram above. It's such a concise description of the important type terminology I learned in design school that I recommend it as a quick and useful read.
The terms we encounter most in the online beginning Adobe Illustrator class I teach include descender, ascender, and baseline. These terms are helpful for creating type on a path and for understanding the settings available in the type on a path options dialog under the type menu in Illustrator.
My next online beginning Illustrator class will start in October on ReneePearson.com, so stay tuned for more information on that. And if you aren't brand-new to Illustrator, you may find my current workshop Swirl Shop useful - we delve into some great Pen Tool and Spiral Tool work in that class - so check it out!