The Allegheny County Employees' Retirement System (ACERS) was established by the county commissioners in 1915 to provide a defined benefit plan to vested employees at retirement. Each county and airport employee shall be required to become a member of the county employees' retirement system within six months from the date of their employment. The said employee may elect to become a member of the retirement system at any time during the aforesaid six months period of time.
Investopedia's explanation of a defined benefit plan is very informative. For information about the specific county retirement plan, refer to the county code link above.