Interest/return: If you save money at a bank, you get interest. The interest rate is often stated as a percentage. In total, you earn the amount saved times the interest. If you borrow money, you also pay interest.
Compound interest, or interest-on-interest: For long-term savings, you receive interest on the interest you received in previous years. Through the compound interest principle, your savings grows faster the longer you save. Try it out at the compound interest calculator.
Opportunity cost: When you buy an item, it means that you opt out of other options. The value of the best option is called the opportunity cost. This is what you have to give up to buy the item.
Sunk-Costs: Ignoring sunk costs is a common decision-making mistake. Sunk-cost means costs you have already had and which should not play a role in how you shop in the future. These costs should not affect the decisions you make.
Mental accounts: Many people divide their income, expenses and savings into different mental accounts. This means that the money gets different labels depending on what it is to be used for. This can be an effective way to get an overview of your personal finances.