This answer deals only with Federal Taxes in the United States.
Many individual states, counties, and cities have their own income taxes, payroll taxes, sales taxes, property taxes, etc., you will need to consult your state and local government websites for information about additional taxes that apply based on your locale.
Wages, Salaries, Tips, Cash bonuses and other taxable employee pay, Strike benefits, Long-term disability, Earnings from self employment
Earned income is subject to payroll taxes such as:
- Social Security:
If you are employed you pay half at 6.2% and your employer pays an additional 6.2% up to the 2017 limit of $127,200 and money you earn above that amount is exempt from this tax. If you are self employed you pay both halves for a total of 12.4%
- Medicare: Same as Social Security except at the rate of 1.45% each for employee and employer (2.9% for self-employed individuals) and without the limit on earnings. In fact, for single individuals making over $200,000 ($250,000 for married filing jointly and $125,000 for married filing separately) there is a bonus 0.9% taxed for the magical 3.8% number that is called Net Investment Tax and has its own IRS form 8960. This additional 0.9% was introduced to help pay for the Affordable Care Act. This tax applies to all income including Long-term capital gains. It's called the Net Investment tax because the majority of the revenue comes from Long-term capital gains.
Earned income is also subject to income taxes which are progressively higher depending on the amount earned minus tax credits, exemptions, and/or deductions depending on how you file. There are 7 tax rates that get progressively larger as your income rises but only applies to the income in each bracket. 10% for the first 18,650 (2017) through 39.6% for any income above 470,700. The full list of rates is in the above linked article about payroll taxes.
Earned income is required for contributions to an IRA. You cannot contribute more to an IRA than you have earned in a given year.
Ordinary (Non-Earned) Income
Interest, Ordinary Dividends, Short-term Capital Gains, Retirement income (pensions, distributions from tax deferred accounts, social security), Unemployment benefits, Worker's Compensation, Alimony/Child support, Income earned while in prison, Non-taxable military pay, most rental income, and S-Corp passthrough income
Ordinary income is taxed the same as earned income with the exception that social security taxes do not apply. This is the "pure taxable income" referred to in the other linked question.
Dividends paid by US Corporations and qualified foreign corporations to stock-holders (that are held for a certain period of time before the dividend is paid) are taxed at the Long-term Capital Gains rate explained below. Ordinary dividends like the interest earned in your bank account are included with ordinary income.
Stocks, Bonds, Real estate, Carried interest -- Held for more than a year
Income from assets that increase in value while being held for over a year. Long term capital gains justified by the idea that they encourage people to hold stock and make long term investments rather than buying and then quickly reselling for a short-term profit.
The lower tax rates also reflect the fact that many of these assets are already taxed as they are appreciating in value. Real-estate is usually taxed through local property taxes. Equity in US corporations realized by rising stock prices and dividends that are returned to stock holders reflect earnings from a corporation that are already taxed at the 35% Corporate tax rate. Taxing Capital gains as ordinary income would be a second tax on those same profits. Another problem with Long-term capital gains tax is that a big portion of the gains for assets held for multiple decades are not real gains. Inflation increases the price of assets held for longer periods, but you are still taxed on the full gain even if it would be a loss when inflation is calculated.
Capital gains are also taxed differently depending on your income level. If you are in the 10% or 15% brackets then Long-term capital gains are assessed at 0%. If you are in the 25%, 28%, 33%, or 35% brackets, they are assessed at 15%. Only those in the 39.6% bracket pay 20%.
Short Term Capital Gains
Capital assets sold at a profit held for less than a year
Income from buying and selling any assets such as real-estate, stock, bonds, etc., that you hold for less than a year before selling. After adding up all gains and losses during the year, the net gain is taxed as ordinary income. Collectibles held for more than a year are not considered capital assets and are still taxed at ordinary income rates.