ERA+
Adjusted ERA is often abbreviated to ERA+ (ERA Plus)
Useful for multiple reasons, including how easy it is to apply.
A metric developed by Baseball Reference that is calculated, as follows:

League Average ERA+ is set to 100 and anything above or below that suggests a pitcher is better or worse than MLB Average, respectively.
ERA-
FanGraphs also uses a similar metric they’ve developed, known as ERA- (or ERA Minus). Check out their summary of the metric:

While ERA+ & ERA- are calculated differently, they answer the same question:
“How good is a pitcher at preventing runs when we account for park & league effects?”

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MAJOR DIFFERENCE
ERA+: The better the pitcher is at preventing runs, the HIGHER this number goes
ERA-: The better the pitcher is at preventing runs, the LOWER this number goes (thus, it can be assessed similarly to a traditional ERA i.e. the lower, the better).
***In both cases, MLB Average is set to 100.
If you would prefer to stick to TRADITIONAL ERA, the full summary for that is HERE!!