Tumor grade 

A tumor grade relates to its degree of change: how abnormal and disorganized its cells look under the microscope and how fast they are dividing. The grade of a tumor tells how fast and aggressive its development will be.

It goes from 1 to 3 (although in some systems can go higher) 
The smaller the grade, the less aggressive the tumor. 

Grade 1 - cancer cells still similar to healthy cells and growing slowly

Grade 2 - cancer cells look different and grow quicker than healthy cells

Grade 3 - cancer cells look very abnormal and divide very fast   

Doctors resort to tumor grading to decide on the best treatment. For example, for tumors at the same stage (of disease evolution), those with a higher grade might need a more aggressive treatment.