What is aggressive gallbladder cancer?

Depends. The microscopic analysis of the tissue can be categorized by a grading system of factors that suggest whether a tumor is likely to be more or less "aggressive;" high grade and low grade, respectively. The stage of the cancer is the most important prognostic factor in term of outcome. Read more about it here: http://www.Cancer.Gov/cancertopics/pdq/treatment/gallbladder/patient.
Gall bladder cancer . Gall bladder cancer can be locally advanced involving other surrounding organs or spread to blood stream to other organs. Prognosis in this situation is poor.