is the term given to developmental reading disabilities. 

In contrast to popular ideas, dyslexia is not necessarily a visual problem and is not diagnosed by the presence of reversed numbers or letters.
Instead, dyslexia is marked by significant reading dysfunction & includes different types as well as combinations of these major types.

surface dyslexia is the inabillity to recognize sight words easily and fluently.  Reading is marked by halting and slow reading where the student tries to 'sound out' each word.  Words that do no follow a phonetic rule (phonetically irregular words) are confusing to this person.  Spelling errors occur as the student endeavors to spell words with phonetic rules that do not consistenly work in our language.  These dyslexics have not developed automatic reading by knowing the shape of the word.

dysphonetic dyslexia is the inability to understand words according to a phonetic code.  This person tries to memorize sight words but has no key to sounding out or building words.

Further reading & reference:  Feifer, S & DeFina, P.  
The Neuropsychology of Reading Disorders  Middletown, MD: School Neuropsychology Press, 2002.