Here is the information I got from the online encyclopedia Wikipedia about Passive Income. As you will note, there are many ways to build a passive income.
You may also note that for those of us that are not artistically gifted such as writers and actors, or who do not have a great deal of money with which to start
a business or buy a lot of income producing real estate, network marketing is also mentioned as a method of building passive income

So here is the exact wording from the Wikipedia site about Passive Income

"There are three types of income—earned, portfolio and passive income. Most types of passive income are derived from rental/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, without continuing effort. This does not mean no effort at all; most passive income streams require great effort to start with.

Passive Activity Income- is defined to include all income generated from rental real estate investments-- regardless of how active the investor is in managing the property or properties.(Real Estate Principles A Value Approach 2nd edition pg.519)

[edit] Examples of Passive Income

• Income from rental property
• Royalties from writing a book.

or offering a service that doesn't require the merchant's intervention. In these cases, PayPal IPN may be used to achieve this.

Another example of passive income comes from network marketing.

[edit] Alternatively, the following are Examples of Portfolio Income

• Interest Income paid from bank deposits.
• Dividends from shares holding
• Profits from stocks or bonds"

[edit] References

• Definition from Investopedia
• Genius Types - 5 ways to make passive income
• Describing passive income and cash flow patterns

Passive Income according to Wikipedia