this chap Dean Rleck (into marketing and copy) has articulated quite well a summary of principles of legibility. rather than ping you off - I will print them below, and put a link up for more if you wish. I am not the author, nor wholly agree, but it is a good foundation and glossary of terms.
The Reading Process
To understand legibility and how design affects readership — and thereby sales — we must first understand how people read. (Forget how people “should” read. We are only concerned with how the average person actually does read.)
Here are the basics of the reading process:
Eye Rhythm — In Western culture, we print written materials with the words arranged horizontally left to right. To read this material, the eye moves left to right along a line of type and then sweeps to the left and down to the beginning of the next line.
Fixations — As the eye moves along a line of type, it stops at certain points to allow the eye to see and the brain to comprehend one or more words. These stops are called fixations, jumps, or “saccades” from the French saquer meaning to pull. Each fixation is about 1/4 second.
Eye Span — During each fixation, the eye sees the word or words upon which it fixates as well as an area around that point. A reader’s eye span may be as small as a single word or as large as whole phrase. A good reader will see about 2 1/2 words per fixation, but the average reader may see less. The ordinary radius maximum is 2 inches around a fixation point. With standard text, this translates to about 29 letter spaces, 17 of which are clearly seen.
Thought Units — The eye span isn’t arbitrary. The brain naturally divides sentences into thought units or idea chunks. In the sentence “Bill caught the ball,” the two thought units are “Bill” — the person who did something — and “caught the ball” — the thing he did.
Configuration — Every word has a particular shape. With constant, repeated exposure to a word, reading stops and instant recognition begins. When you see a stop sign, for example, you don’t mentally sound out S-T-O-P, you perceive the whole word by its shape and instantaneously understand its meaning. Because numerals have little configuration, the eye fixates more on numbers than words. Also, since there are more shape differences with lower case letters than with capitals, configurations in lower case are recognized faster than all caps.
Recognition Rate — How fast a reader understands words during reading is called the recognition rate (or word response rate or rate of perception). Obviously, the faster a person recognizes a word, the faster and more effortless the reading. In the previous sentence, you probably whizzed over words like “the” and “a,” but paused for a split second on “recognizes” and “effortless.”
Familiarity — The more familiar a reader is with the type and the appearance of the words used, the easier the reading. Roman or serif faces are generally more familiar to readers. Type that gives words irregular features give words a more distinct and recognizable shape.
Reading Rates — The average person shows a constant increase in reading rate throughout the school years, followed by a sudden drop after graduation. In Junior High, the average reading rate is 200 words per minute (wpm). In High School, it’s 250 wpm. In College it rises to 325 wpm and then to 400 wpm in Graduate School. Then it drops back to 200 wpm in adulthood, with reading comprehension at about 50%.
Compare this to the average rate of speech, which is 140 to 160 wpm, and you’ll see that most people read slowly. (By the way, for most people, anything above 600 to 700 wpm is scanning, not reading.)
Regression — Moving the eye back over previously read material is called Regression. Not only does this lower the reading rate, it actually alters the sequence of information into the brain and lowers comprehension.
Eye Fatigue — The average adult eye travels approximately 1,600 feet per day — that’s 584,000 feet or 110 miles per year! So, it doesn’t take much to produce Eye Fatigue, which in turn slows the recognition rate and generally makes reading slow and even more difficult.