What is legibility?
It is the ease at which someone can recognize type on a page. Lowercase text is easier to read than uppercase text because our eyes can identify with the variation in shapes. Research has show that our eyes skim the top half of the letters and if the text is legible we quickly can process the information. 

What is readability?
A typeface has to be legible before it can be readable. Text readability is when a person can process text quickly and comfortably without interruption.

What is information hierarchy?
When designing a web page it is important that you plan how your information will be presented and what is most important. You can establish importance by using type size, style and spacing to form a hierarchy of information. If this plan is consistently used readers have an easier time navigating, understanding and learning from your website.

Hierarchy suggestions
Printed body copy is usually set between 6pt and 12pt type. I do not recommend that you use less than 10pt type on the web. The screen glare and type size can cause  eye fatigue pretty quickly. When setting up your type hierarchy, choose a font with a moderate x-height. Readability will improve if you have control of your line length, hyphenation and leading. Use bullet lists to highlight key points. Illustrations, photos and animations can be added only if they support the contents message.

Anatomy of a Typeface