An air mass, by definition, is a large dome of air which has similar horizontal temperature and moisture characteristics. As you can see in the graphic above with hypothetical air masses, the temperature regime in the large cold air mass over the Plains is very uniform. In other words, the temperature does not change much from Helena, Mont. to St. Paul, Minn., which are both in the cold air mass. The same concept holds true for Columbus, Ohio, to Atlantic City, N.J., which are both in the warm air mass. Often, a front separates two different air masses.
Fronts are very narrow zones of transition.
In other words, temperatures can change dramatically with short horizontal distances near fronts. The graphic above shows this concept with frigid temperatures over Indianapolis, Ind., and much warmer temperatures in Columbus, Ohio. Columbus and Indianapolis, Ind., however, are relatively close to each other. Fronts are usually anywhere from 10 kilometers to hundreds of kilometers wide, while air masses can be thousands of kilometers wide.