Schedulers is certainly one of the most important component of Quality of Service (QoS) on Juniper Junos. Schedulers defines the queue parameters your traffic will be subject to. Each queues can receives different scheduling parameters thus allowing for service differentiation.
I cannot cover Diffserv without ever mentioning his predecessor, the Integrated Services model or Intserv. The latter never seen global deployment because of scalability issues inherent of its architectural design. The idea behind Intserv was to provide end-to-end QoS guarantees to applications like voice, video and conferencing.