There are many different kinds of slavery, but in basic terms, the practice involves owning someone else in some manner. These owned individuals are called slaves, and they usually have a limited amount of choice about what kinds of actions they can take and are normally forced to perform some task for another person without any reasonable compensation. In general, a slave becomes another person's property, and he can be used by that person for financial or personal gain. Slavery is illegal in many parts of the world now, including the US, but there are certain forms of slavery that are still commonly practiced.
In ancient societies, a lot of slavery was based around using captured soldiers during war as forced laborers. In some cases, these prisoners were kept as slaves for the rest of their lives, and in other situations, they were allowed to go free after a certain point. Sometimes slaves were actually treated quite well in these sorts of arrangements, but it varied a lot from once society to another. Keeping soldiers as slaves is still practiced in some cultures, with prisoners of war being forced to perform different kinds of physical labor, but in ancient times, it was much more common.
Chattel slavery is generally the most well-known form practiced historically. Chattel slaves are treated like property, and they can be sold or bought. In most cases, chattel slaves are usually chosen based on racial or geographical divides. This is partly because of the requirement for the complete dehumanization of another population, which is generally easier to justify with individuals that are deemed physically or culturally different. This is the kind of slavery that was formerly practiced in the US prior to the Civil War.
Debt bondage is another form of slavery that is still practiced around the world. This happens when a person gets a loan and gives up her freedom as a form of collateral. Sometimes parents even sell their children into this kind of bondage. In theory, debt bondage ends when the loan is paid off, but in practice, the deal is almost never so simple, and people often end up with impossible debts to pay.
Sex slavery is another form that is still practiced commonly. Often, women are kidnapped or tricked and forced into prostitution. In some parts of the world, women have so few rights that their husbands or parents can push them into this kind of life. Sex slavery is illegal in most countries, but it happens under the radar in the criminal black market. Some people consider forced marriage a form of sex slavery and in some societies, it was historically common for women to be treated like slaves by their husbands.