Dyslexia comes from the Greek language meaning 'difficulty with words'. It's a symptom of a number of different information processing disorders in the brain.
Because there are so many different possible underlying problems (many of which have yet to be understood fully) dyslexia is hard to closely define because it affects children in many different ways. However, the basic problem is a difficulty learning to read, spell and write.
Visual dyslexia is the term used for the specific learning disability termed visual processing disorder. This form of dyslexia is the result of immature development of not only the eyes, but the whole process that gets information from the eyes to the brain. Eyes that are not completely developed will send incomplete information to the brain. Incomplete information to the brain then results in poor comprehension of what the child has read, or poor memory of visual information. Sometimes this process results in number and letter reversals and the inability to write symbols in the correct sequence.
Phonological (auditory) dyslexia refers to the specific learning disability termed auditory processing, or the more severe condition termed Auditory Processing Disorder (OPD). This form of dyslexia involves difficulty with sounds of letters or groups of letters. When this form of dyslexia is present, the sounds are perceived as jumbled or not heard correctly. And just as with visual processing, the brain correctly interprets information that it correctly received.
Dyspraxia refers to the learning disability term sensor-motor integration and is a widely pervasive motor condition characterized by impairment or immaturity of the organization of movement, with associated problems of language, perception and thought. Typically, the child in question may be seen to be clumsy and poorly coordinated.
Dysgraphia is the term given to the most significant educational effects of the condition and refers to an inability to hold or control a pencil so that the correct markings can be made on paper. These symptoms are most commonly seen as poor letter formation in printing, or as poor cursive handwriting skills. As a specific learning disability these symptoms would be identified as immature fine motor development.
Dyscalculia is a lesser known disability, similar and potentially related to dyslexia and developmental dyspraxia. The term refers to an impairment of the ability to solve mathematical problems, usually resulting from brain dysfunction. It occurs in people across the whole IQ range, and sufferers often, also have difficulties with time, measurement, and spatial reasoning.