ACAS is the Advisory, Conciliation and Arbitration Service, it is a public body funded by the British taxpayer whose function is to prevent and resolve problems in the workplace. In 1976 it was recognise as a statutory body under the terms of the Employment Protection Act 1975.
It was founded as an independent public body in 1974 when it was known as the Conciliation and Advisory Service with the the "Advisory" bit being added a year later, but its roots stretch back to the voluntary arbitration and advice service set up by the British government in 1896 that later became known as the Industrial Relations Service in 1960. It exists to give free advice to employers and unions on industrial relations issues and provides a dispute resolution service, not only for collective union-employer disputes but also for individual complaints regarding employers that would otherwise be dealt with by employment tribunals. It also seeks to promote "good practice" through the all the usual methods of promtion.
See http://www.acas.org.uk