Quality of Service is the general idea of prioritising, policing, shaping traffic and dealing with Queues.

QoS? covers things like prioritising some traffic (Usually Latency sensitive traffic such as Voice, and control traffic) over some other traffic, it can cover delivering a lower quality of service to a user perhaps because they haven't payed for a higher one. (eg, rate limiting users to 128k/128k instead of letting them use 2M/512k).