Preliminary Microeconomics Concepts

Economics: study of how people use and allocate scarce resources
Opportunity cost: best choice that you give up when making a decision. Can't have everything we want because time and resources are scarce. Eg. watching TV for 3 hours when you've got exams coming up means an opportunity cost of 3 hours worth of study that's given up.
Marginalism: you're supposed to make economic decisions based on weighing up the costs and benefits only from the decision itself. Eg. only consider marginal benefits (additional benefits arising  from decision) and marginal costs (additional costs arising from decision). Ignore sunk costs (costs that have already been incurred in the past so won't make a difference one way or another).
Efficient markets: everyone has access to relevant information about products and services and their prices etc so profit opportunities quickly disappear. If you buy something because of some public information about it other people  would also have access to that information so price would be fair and you can't make a consistent profit.

Difference between microeconomics and macroeconomics
Microeconomics is the study of how individual people, businesses, families and industries make economic decisions whereas macroeconomics is the study of the economy as a whole like studying the whole forest instead of just the trees.