Wikipedia describes differentiated services or DiffServ as "a computer networking architecture that specifies a simple, scalable and coarse-grained mechanism for classifying and managing network traffic and providing quality of service (QoS) on modern IP networks". Read more here: http://en.wikipedia.org/wiki/Differentiated_services.
A bit of history on Type of Service (ToS), IP Precedence, and DiffServ (DSCP) can be found here: http://en.wikipedia.org/wiki/Type_of_service
Essentially, both DiffServ/DSCP and IP Precedence are used to classify and differentiate traffic in a Quality of Service enabled network. The relationship between DSCP and IP Precedence is detailed in the table below.