A Biological is a protein that blocks another protein (cytokine) that plays an important role in the inflammation of some rheumatic diseases. The most well known are TNF and Interleukin 6 (IL-6). Not always, biologics block cytokines. They may also target cells that have an important role in inflammation or block the interaction of different cells, which, stimulates inflammation. The fundamental difference with other therapeutic agents is that most of the drugs are chemical compounds. However, biologics just for being proteins have a very complex structure and higher molecular weight, and they are degraded in the stomach quickly, which, prevent to administer orally. Therefore, biologics are only given intravenously or subcutaneously (such as insulin).
Note: Biological are not exclusive treatments for rheumatic diseases, other diseases, such as cancer and many others, also have biological treatment
See section Biologics, to find the topic in extend