Hamas is one of the two richest terrorist organizations in the world, second only to the Islamic State group, according to Forbes Israel.

To finance their operations, terrorist groups sometime use methods similar to those used by criminal organizations, such as drug trafficking, robberies and extortion, but also raise money through charities, donations and, in some cases, government agencies, according to the report. A terrorist organization, like any other large organization, has a business model to finance its activities: from maintenance, salaries and trainings, to acquiring of weapons and vehicles.

The richest terrorist organization today – and in history – is the Islamic State. According to Forbes, the Islamist group, also known by the acronyms ISIS or ISIL, has an annual turnover of $2 billion. Hamas comes in second, with a yearly revenue of $1 billion.

As for the rest of the list: Colombia's FARC is ranked third with $600 million; Hezbollah is fourth with $500 million; fifth is the Taliban with $400 million; Al-Qaida and its affiliates with $150 million; Pakistani-based Lashkar e-Taiba with $100 million; Somalia's Al-Shabaab with $100 million; Real IRA with $50 million; and, closing the top-ten list is Boko Haram, with $25 million annual revenue.

The U.S. Treasury estimates that ISIS earns $1 million a day from the sale of crude oil from fields it captured in Syria and Iraq. According to Forbes Israel, however, the figure is closer to $3 million a day. The money flow enables ISIS to expand its operations in the Middle East, recruit foreign fighters and train them, among other things.

The report describes Hamas' takeover of Gaza in 2007 as the point from which it entered "the big league." Today, Hamas is now no longer only dependent on donations, but is able to collect taxes from both civilians and businesses. But that's not all. The report says Hamas also takes a cut out of all international aid donated to Gaza by Arab and other foreign countries.