Nicki,

The term "differentiation" is used to describe the structure and rate of growth of tumor cells. Well-differentiated cells tend to resemble normal, mature cells and grow rather slowly. Poorly differentiated cells don't have the same structure as normal cells and tend to grow faster. (I'm paraphrasing from a glossary on the NCI website -- you can find it under "Other Resources" on this site, listed in the Medical Databases category.) I've found it helpful many times for looking up the technical terminology that doctors toss around during the diagnosis and treatment process.

Cathy