According to the British Dyslexia Association (BDA) dyslexia is:
"... a specific learning difficulty that mainly affects the development of literacy and language related skills. It is likely to be present at birth and to be life-long in its effects. It is characterised by difficulties with phonological processing, rapid naming, working memory, processing speed, and the automatic development of skills that may not match up to an individual's other cognitive abilities."
Here are two great videos that also explains all about dyslexia.
YouTube video by Mike Juggins
YouTube video by TedEd.