Reham - I am not sure if you have a specific question about how multi-drug resistant bacteria evolve, or how they function, or types of them. If you are more specific, I am sure you will get answers that are more useful to you.
In general we define a multi-drug resistant bacteria culture as one in which the bacteria is resistant to several antibiotic classes (or important members of a class that function differently). This is for those antibiotics that should be able to kill or stop growth of other members of the same species/strain. Because of the inability of several major groups of antibiotics to kill the resistant form, it may be able to grow out of control and cause disease.