To put it simply, the main difference between the two is that with a full-service payroll, an external company is managing the payroll services unlike a self-service where a person or team within the company manages the payroll.  During self-service payroll, there is often an accountant or members of the HR team who work in an office in the company to fill tax forms, pay slips etc. On the other hand, with full-service there are no direct employees. The chosen payroll company is paid as per the contract and the work will be done by a team hired by the given company.