This baseball statistic represents the amount of earned runs, on average, that a pitcher allows for every nine innings he or she pitches. A pitcher's ERA is one of the most fundamental stats used to compare pitchers. ERA is calculated by dividing the pitcher's earned runs allowed by the total number of innings pitched and multiplying that total by 9.
Earned Run Average = 9 x (Earned Runs Allowed/Innings Pitched)
For the most part, the ERA is a fair way to look at pitchers without their performances being negatively affected by their team's defense, because earned runs allowed focuses on those runs that are attributed to the pitcher.
Comparing ERAs across the many different "baseball eras" is difficult due to the changes the game has undergone over time. Prior to the turn of the twentieth century, pitchers had ridiculously low ERAs based on the pitcher's mound being much closer to the batter. In contrast, it's rare these days for a starting pitcher to have an ERA in the low 2.00s. The average is considerably closer to 4.50, and those posting an ERA that low are a special type of pitcher.
It is also important to note that ERA fluctuates a great deal based on a pitcher's place in the rotation or the division in which he plays. For example, the best relief pitchers in the game may have high ERAs, for when they get beat up so does their ERA. If a relief pitcher gives up two runs in the ninth in his first appearance as a reliever, he will spend the rest of the year trying to bring his ERA down from 18.00. Teams also play within their divisions so often that a division having a bad offensive year may artificially show pitchers as more dominant.