Define Micro Economics. Shows the use of microeconomics in business decision making?

Microeconomics is the very specific study of a single situation (as compared to macroeconomics which doesn't care about specifics and doesn't want to know about individuals, just averages and trends for whole populations).

Where macroeconomics suggests we all fit a happy average, microeconomics is about making the fit right for one person. Macroeconomics suggests that we all wear size nine shoes and anyone with a different size will just have to suffer badly fitting shoes. Microeconomics measures our individual foot size and gives us (where relevant) a size 9.456374 shoe. Or a size 14.35465 shoe in the case of someone with big feet.