March 14 is #MyFreedomDay, when schools around the world will raise awareness of modern slavery. Find out more at cnn.com/myfreedom

(CNN) The CNN Freedom Project has been reporting on modern slavery since 2011, but even now, one of the questions we're asked most often is: What is modern slavery?

There's no one universally accepted definition, but the one used by CNN is: "Slavery occurs when one person completely controls another person, using violence or the threat of violence to maintain that control, exploits them economically, pays them nothing and they cannot walk away."

Slavery has existed for thousands of years. What's different about slavery today is that it's illegal, and as a result it's more hidden than in the past. Nonetheless, it's thought that there are more slaves today than at any time in history.

How about human trafficking?

The term "human trafficking" is sometimes used interchangeably with modern slavery.

The U.N. Trafficking Protocol defines human trafficking as: "the recruitment, transportation, transfer, harboring or receipt of persons, by means of the threat or use of force or other forms of coercion, of abduction, of fraud, of deception, of the abuse of power or of a position of vulnerability or of the giving or receiving of payments or benefits to achieve the consent of a person having control over another person, for the purpose of exploitation."

How big a problem is it?

There are around 40 million people living in modern slavery , according to the Global Slavery Index 2017, although it's difficult to know for sure, because traffickers want to keep them hidden from view.

Children make up an estimated 10 million victims , according to the index.

And it's big business. Globally, modern slavery is worth around $150 billion a year , according to the International Labour Organization (ILO).

Types of modern slavery

According to the Global Estimates of Modern Slavery and the US State Department, modern slavery comes in a number of forms:

Sex Trafficking

When someone has been forced into commercial sexual exploitation, or when they cannot leave, whether by physical restraint or violence or threat of violence to them or their family.

Women and girls account for 99% of sex trafficking victims.

Child Sex Trafficking

Whenever a child (someone under 18 years of age) is used for a commercial sex act. With a child, there doesn't need to be any force or coercion involved.

Forced labor

When someone is forced, coerced or tricked into providing labor, and they cannot leave. It could be picking fruit , toiling in a factory or working in a nail salon. Debt is often used to keep victims in forced labor -- a practice sometimes known as bonded labor.

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Bonded Labor/Debt Bondage

Bonded labor is when an employer forces someone to work to pay off a debt. Sometimes, workers are charged recruitment fees by an agent, other times they have to pay off a loan.

They are often charged exorbitant interest rates, making the debt impossible to pay off. These debts can be passed on from one generation to the next, as can be the case in South Asian brick kilns

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Domestic Servitude

Domestic workers, particularly migrants, are especially vulnerable to exploitation , because they work out of sight in private homes. It's considered slavery when they're not free to leave and aren't paid, or are underpaid.

Often, migrant domestic workers have their passports confiscated by their employers. They can be isolated, with no protection against abusive treatment -- which can include violence.

Forced Child Labor

According to the International Labour Organization , "child labor is often defined as work that deprives children of their childhood, their potential and their dignity, and that is harmful to physical and mental development."

It interferes with a child's ability to go to school, and in extreme cases "can involve children being enslaved, separated from their families, exposed to serious hazards and illnesses and/or left to fend for themselves on the streets of large cities."

Forced marriage

Forced marriage is where someone, regardless of their age, is made to marry against their will. At least 15 million people globally are living in a forced marriage, in a situation where "they have lost their sexual autonomy" and often are providing labor, according to the ILO.

Child Soldiers