Terminology
Although their use was first propagated in rheumatoid arthritis (hence their name) the term has come to pertain to many other diseases, such as Crohn's disease, lupus erythematosus (SLE), immune thrombocytopenic purpura (ITP), myasthenia gravis and various others. Many of these are autoimmune disorders, but others, such as ulcerative colitis, are probably not (there is no consensus on this).
The term was originally introduced to indicate a drug that reduced evidence of processes thought to underlie the disease, such as a raised erythrocyte sedimentation rate, reduced haemoglobin level, raised rheumatoid factor level and more recently, a raised C-reactive protein level. More recently, the term has been used to indicate a drug that reduces the rate of damage to bone and cartilage. DMARDs can be further subdivided into traditional small molecular mass drugs synthesised chemically and newer 'biological' agents produced through genetic engineering.
Some DMARDs (e.g. the Purine synthesis inhibitors) are mild chemotherapeutics but use a side-effect of chemotherapy - immunosuppression - as its main therapeutical benefit.
Read more about this topic: Disease-modifying Antirheumatic Drug