There are three types of income - earned, portfolio and passive income. Most types of passive income are derived from real estate/property, while other types of passive income are derived from royalties from patents or license agreements.
An income stream falling into this category, is one where money is received usually on a regular basis, where no additional effort has taken place. This does not mean no effort at all. Most passive income streams require great effort to start with.
Some examples: Interest Income paid from bank deposits. Rental income from real estate/property. Royalties from writing a book.