Over the long haul, a big part of successful investing is building a mental database of companies and industries on which you can draw as the need arises. The list below should give you a jumpstart in compiling that mental database, and that will make you a better investor.
It is easier for companies to make money in some industries than in others. Some industries lend themselves to the creation of economic moats more so than others, and these are the industries where you'll want to spend most of your time. The economics of some industries are superior to others. Hence, you should spend more time learning about attractive industries than unattractive ones. Every industry has its own unique dynamics and set of jargon - and some industries (such has financial services) even have financial statements that look very different.
Ref: The Five Rules for Successful Stock Investing by Pat Dorsey