"Concentration" measures are indicators of the state of an industry in terms of the existence and possible dominance by larger firms or by smaller firms. Airlines for example is a highly concentrated industry; beauty shops is a very low-concentration industry.
"Concentration Ratios" are various calculations that provide a measure of concentration of industries. There are several different calculations and the distinctions are not particularly important for this present discussion. In general the different calculations are consistent in their results as to indicating which industries are relatively more/less concentrated than others, and the extent of the concentration.
