Excerpt from About.com: (click for entire article)
Depending on the educational jurisdiction, the term Dyslexia may or may not be used to define a disability in reading. Some educational jurisdictions refer to Dyslexia as a type of learning disability pertaining to reading. However, Dyslexia is often described as a reading disorder or a disability affecting reading or the impairment of the ability to read. Typically, those with Dyslexia have a normal intelligence. The prevalence of Dyslexia has been estimated to occur in between 5-10% of the population. Dyslexia symptoms can range from mild to severe.
Categories Affected by Dyslexia
Dyslexics will often confuse letters (b for d, p for b etc.) and say words backwards (tac for cat) and will often omit simple words in sentences. Word and letter reversals are common as are the omissions of small words and parts of words. Their ability to see the sequence in words or sentences is somewhat scrambled. Their visual and phonetic memory is extremely weak which causes the deficits in reading.