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DSCP is a way of marking packets so they may easily identified for different service level treatments.
QoS is the overall "bucket" that encompasses providing different service level treatments.
Basically some traffic, such as packets from VoIP phones have their packets marked differently so those packets might be treated better, when there's resource contention, on switches (and routers).
As different DSCP marking might be used for different service levels, the issue of trust arises. I.e. is a marking that denotes better treatment "trusted" to be legitimate. A trust boundary defines demarcation of a range, within a network, that a DSCP marking is assumed to be valid.
QoS isn't really complex, but it's very broad subject. You need to know much to understand how it all fits together.
Cisco has some nice information on QoS, you might start with documents off this page: http://www.cisco.com/c/en/us/solutions/enterprise/design-zone-application-performance/landing_cVideo.html
