Para-Dressage is an equestrian discipline that is one amongst many in para sports aka disabled sports. These are sports played and conducted by people with physical impairment. And when we talk about para sports we usually mean those sports you can see on Paralympics. Para-Dressage is one of them or to be precise Para-Equestrian Dressage.
The Fédération Equestre Internationale (FEI) is the controlling authority for all international events in various equestrian disciplines and Para-Dressage is one of them amongst Dressage, Jumping, Eventing, etc.
Para-Dressage is lot like Dressage with few exceptions. It differs from Dressage in couple of ways.
In Para-Dressage every athlete has a physical impairment and is classified according to the level of their impairment. FEI has a really thorough Classification Manual and athlete's physical impairment are assessed in order to establish their Classification Profile.
Athletes with similar functional ability Profiles are grouped into competition Grades. The Grades range from Grade I for the most severely impaired, to Grade V for the least impaired. And the Grade defines the test athlete rides in competitions. Grade I athletes perform only in walk. Grade II and III tests include walk and trot. Grade IV and V tests are performed in walk, trot and canter. Athletes are permitted to use special equipment when this has been formally documented and approved.
Para-Dressage differs from Dressage slightly on how it's been judged. For example Rider's position and seat are not being scored. Some riders may have severe errors in their position but are still able to ride perfectly. It's all about influencing the horse and that is what the judges are grading in tests.
FEI released Guidelines to judging Para-Equestrian Dressage movements in October 2017. Document is in FEI's website. (pdf, 5 pages)
Para-Dressage athletes follow the General rules, Dressage rules and Para-Equestrian Dressage rules.
FEI was one of the first International Federations to govern and regulate a sport for both able-bodied and athletes with impairments.