IncomeFinancial Dictionary -> General Finance -> Income
Income does not factor in any costs of living that you may have, for example you may make $700 a week but $30 is gone straight away for the electricity bill. No matter what your costs your income was still $700 for that week. However your disposable income (income that has the ability to be spent on leisure) would be $670.
By most laws around the world all income is subject to tax, whether it was made from your main job or not. Usually if you are employed your employer sorts out all the tax before you ever get your paycheck. If you are self employed you have to do your own taxes, with any of your business costs offsetting the taxable income. Although it is technically illegal, any odds and ends you may do for cash, or any random item that you sell on Ebay can be ignored in regards to tax. Any money you receive as a gift is not considered taxable.
A person's wealth is often measured by their average monthly income, although that doesn't factor in how much they spend. Somebody may make $10,000 but spend $9,000 on jewelry and clothes, whereas somebody else may make $5,000 and only spend $500 - it doesn't necessarily make them wealthier.