To define differentiated services (DiffServ), we'll defer to the experts at the IETF. The following
excerpt is from the "Abstract" section of RFC 2475:
This document defines architecture for implementing scalable service differentiation in the Internet. This architecture achieves scalability by aggregating traffic classification state which is conveyed by means of IP-layer packet marking using the DS [...]

Packet loss is just losing packets along the forwarding path. Packet loss results from many causes, such as buffer congestion, line errors, or even QoS mechanisms that intentionally drop packets.
Table 1-1 shows examples of the varying requirements of common applications for bandwidth,delay, jitter, and packet loss.
Table 1-1. Traffic Requirements of Common Applications
Voice             FTP                Telnet
Bandwidth Required  [...]