Stelara, Remicade, and Enbrel are all biologics -- drugs made of genetically engineered proteins -- that are generally used to treat patients who aren't responding to traditional therapies such as light therapy and methotrexate.
Remicade and Enbrel both block tumor necrosis factor-alpha (TNF-alpha), a chemical produced by immune cells that fuels inflammation, much like gas on a fire. Stelara targets two proteins, interleukin 12 and interleukin 23, that also drive the inflammatory process.
References:
Study Shows Stelara and Remicade Are Both Effective if Enbrel Stops Working. WebMD, 2010.
Image source: Crystal structure of human IL-12, Wikipedia, public domain.