Cancer grade is how cancer cells look under the microscope compared to normal cells. There are three main degrees of differentiation that are used to determine grade, which in turn can help determine how fast the cancer will spread. 

1) Grade 1 or Well-differentiated: Well-differentiated cells look like normal, healthy breast cells. Grade 1 cancers are not as aggressive and spread slower than cells of a higher grade. 

2) Grade 2 or Moderately differentiated: Grade 2 cells look more abnormal that Grade 1 cells, but less abnormal than higher grade cells. Therefore, they are moderately aggressive and spread at a moderate rate compared to the other two grades.  

3) Grade 3 or Well Differentiated Cells: Well differentiated cells are "ugly" cancer cells that do not resemble normal breast tissue cells. These cells have the potential to replicate and spread very quickly.