Psoriatic arthritis, in short called PsA, is a rheumatologic disease that effects about 1,5 million Americans. About one in three people with the skin disease psoriasis, will get psoriatic arthritis. It affects the joints as well as the tendons, and it may also affect the spine.
In most people, PsA starts with psoriasis. Most people develop psoriasis about 10 to 20 years before being diagnosed with psoriatic arthritis. This psoriasis may be mild or extensive. People may also develop nail psoriasis, a condition seen more often in PsA than psoriasis without arthritis."

Source : US Johns Hopkins Arthritis Center

The causes of PsA are unknown. It is generally regarded as an auto-immune dysfunction, which has genetic and environmental links. It has also been linked to obesity and viral/bacterial infections, which can be triggers for its development.

With regard to genetics, one particular factor, Human Leukocyte Antigen (HLA) B27 (associated with a gene found on chromosome 6) has been noted as a possible link, but its not yet clear how HLA-B27 affects progression of the disease.

Also see : Psoriasisplugin-autotooltip__plain plugin-autotooltip_bigPsoriasis

"Psoriasis is a long-lasting autoimmune disease characterized by patches of abnormal skin. These skin patches are typically red, dry, itchy, and scaly. On people with darker skin the patches may be purple in colour. Psoriasis varies in severity from small, localized patches to complete body coverage."