Systemic lupus erythematosus, also called SLE or lupus, is a disease of the immune system that causes the immune system to attack tissues in various parts of the body. The cause of lupus is unknown. Finding the cause is the object of major research efforts.
Source : Cleveland Clinic health library
It's currently suggested that it may be triggered by infection (viral or bacterial) - by environmental factors such as chemical pollution etc. - or by hormonal imbalances.
There also appears to be a weak genetic link. About 10% of lupus patients have a close relative who has, or has had, the condition.
Further reading : Wikipedia