Employees contribute pre-tax dollars with defined contribution plan. The most common DC plans are 401k/ 403b. In this plan, both employee and the employer contribute. The employee contributes to his/her 401k/ 403b and the employer contributes a match.
Your employer may contribute additional money, in addition to the match. My previous employer contributed 3%-11% of the employee’s salary annually. The amount increased with longevity.
You can roll over your retirement savings to another plan if you leave your employer or withdraw from it if you are over 59 ½.