A 401K rollover is a process in which the funds from an employee's retirement account transferred to another business. The most common reasons for the use of a rollover is due to a job change. For example, if an individual works in a business offering a 401k plan and leaves the business for any reason, he or she can roll over the funds from that business's accounts into their new employer's 401k account. Because the individual did not specifically touch the funds from the retirement account, there is not penalty leveled on the plan. If the funds are withdrawn and sent to the individual, those funds may then be taxed a heavy penalty tax. If an individual leaves a business but cannot roll the funds into another business's 401k for some reason, there are other accounts these funds placed in to avoid the taxes.