Basically, wage theft happens when workers don’t get paid for work they completed. No matter what the economic situation of an employer, the Fair Labor Standard Act mandates that for every hour worked by an employee, his or her employer must pay them for work.
Each year, workers in the United States have billions of dollars stolen from them as a result of their employers depriving their employees of legally mandated wages. Low-wage workers are the people most impacted by wage theft. The largest sums of money are missing from the paychecks of native-born, middle-income workers who are not paid what is due to them for working overtime. Wage theft occurs in every income-tax bracket, in every industry, and in every state.

For more information on local wage theft, see this link to a recent historic report on the conditions of Chicagoland workers. The study’s authors estimate that Chicagoland workers are robbed of $7.3 million every week by their eimployers.
http://www.urbaneconomy.org/sites/default/files/Unregulated%20Work%20in%20Chicago%204_7_2010%20FINAL%20REPORT_0.pdf