Meaning of diagnosis

Dear Doctor,
After doing a biopsy, my husband has been diagnosed as having PYODERMA GANGRENOSUM.
This was about a year back.  It developed very seriously on three parts of his body, forehead, under his left arm and on his left nipple.  He has been on steroids for almost two years, but it has tapered off and now he does not take them.  He had a lot of side effects like cataract, stiffness and numbness of his thighs, ankle pain, bloating, etc.
My questions are :-
1.  What is pyoderma gangrenosum in simple language ?
2.  Can it come back again ?
3.  What can be done to prevent this from reoccuring again ?
4.  Is it cancerous ?
Your detailed views on the above will be much appreciated.
Thank you and God bless.