War score is the way in which the current results of a war are measured. It is measured by percentages, and is determined by the number of enemy provinces occupied, battles won or lost and current blockades.
If you occupy all of a countries provinces and colonies, your war score will be 100%, while if the enemy country occupies all of your provinces and colonies, your war score will be -100%. Some provinces give a higher modifier to the war score, the population, province improvements, cores, religion and culture affect the percentage gain if occupied. Also, national capitals give a big war score modifier. Provinces not on the same continent as the capital (e.g., colonies) are worth much less war score. The war score is only used while suing for peace; the higher your war score is, the more you can successfully demand from the enemy country.