Imuran and 6-MP are both immunosuppressants. They put a damper on your immune system. Possible side effects include nausea, sensitivity to sun, fatigue, and liver problems. There's also an association with lymphoma when taken at high doses (roughly 3-4 times what is prescribed for UC) or when taken in conjunction with the biologicals. Liver problems are not common, and lymphoma is very rare.
Remicade and Humira are bilogicals. They work by suppressing a specific component of the immune system, TNF. People can have allergic reactions to either, and may build antibodies to the drugs so they no longer work. There are a number of other possible reactions described, but they happen in a very small number of people. In people who have latent multiple sclerosis, Remicade can activate it. The biologicals can also produce lupus-like symptoms.
Immunosuppressants are oral medications. Remicade is given by IV every 8 weeks, and Humira is injected, I think weekly.
You can get more information from the CCFA web site:
http://www.ccfa.org/info/treatment/medicationsPrednisone is not appropriate for long-term therapy because it has some truly grim side effects from long-term use.