It varies from state to state, but in most states, "tipped employees" have a much lower minimum wage rate than regular employees. In Nebraska, for instance, the minimum wage rate for tipped employees is $2.13 per hour.
Now, the Fair Labor Standards Act requires that all employees receive at least the federal minimum wage ($7.25) for their combined minimum wage rate plus tips, so if someone receives very poor tips their employer must make up the difference. Individual states can set their own higher minimums; Nebraska, for instance, has a minimum wage of $10.50, so any employee who does not make $8.37 in tips must be paid the difference by their employer.
Alaska, California, Guam, Minnesota, Montana, Nevada, Oregon, and Washington require the employer to pay the minimum wage as the base rate, plus tips on top of that.