Income might sound pretty simple, but it actually comes in multiple forms. All this can be considered income -- follow the links to learn more:
Bonuses, commission, or stipend (often these are included in your regular paycheck, but sometimes they are treated differently)
Tangible assets (like your house or car)
Investment assets (like mutual funds. These can also be individual retirement accounts, or IRA's)
Retirement accounts (like your employer's 401K or other plan, often called a pension)