Historically, slaves where people who were owned by an individual or corporation who could be made to work for nothing. However this is not the essence of slavery. Sometimes these people were paid, but they were still slaves. The essence of slavery is that slaves are not free to choose who to associate with - they are forced to work for the benefit of someone else.
It has become popular in modern times to refer to other people as slaves. Workers from poor countries who are guest workers in richer countries (eg as maids for wealthier families) are often labeled as slaves. These people have few skills, have low productivity and are paid a very low wage by the standards of the richer country. This is an example of definition hijacking which ignores the fact that these people are (usually) free to return to their own countries - they are not forced to work for another's benefit, but merely for their own. The fact that they are unable to to earn as much money as other people does not make them slaves.
Some people refer to anyone who works for someone else as a wage slave.