6-MP is Mercaptopurine, or, generically, purinethol. It's an immunomodulator. In other words, it's not a complete immunosuppressant, in that it doesn't try to totally suppress your immune system. It just kicks it down a few notches. The theory (like that used with other immunological drugs) is that, if you have CD, your immune system is in overdrive. If you tone it down, you may be able to relieve inflammation in your intestinal tract--the inflammation that can eventually become scar tissue, cause strictures, and require surgery. 6-MP comes in tablet form (i.e., you just take it with water, usually once a day--no IVs or injections).
The dose varies, depending on many factors, including the severity of your disease, your reaction to the medication, your body weight and general health, etc. Because 6-MP is a powerful drug, you'll undergo periodic blood-testing (generally once every three months, once your dosage is stabilized) to make sure that your body (especially your liver) is tolerating the medication.
Some people experience side effects (I've seen references to hair loss and nausea); other people experience very few, if any.
As with all drugs that affect your immune system, you're supposed to be especially careful to wash your hands and avoid being exposed to harmful pathogens. However, after Hurricane Katrina, I did (and was exposed to) things that should have killed someone with a suppressed immune system, and I was just fine. Believe me, I took a lot of truly terrible risks. My Crohn's has continued to flare, but my stricture has decreased markedly. So 6-MP has been a lifesaver for me, with minimal side effects.
Does that help? I'm sure that other people on this site can give you more and better information. But at least this is a start.