Traditionally, the Best Effort Internet has provided the worst possible service: packets are forwarded by routers solely on the basis that there is any known route, irrespective of traffic conditions along that route. Routers that are overloaded discard packets, typically dropping packets at the tail of the queue of those awaiting to depart along their way.
Other types of digital networks have been built, most notably, for wide public access, the digital telephone network, with user access based on the narrow band Integrated Services Digital Network architecture. This is in fact, the Fixed Effort ISDN, which gives you a constant data rate from source to sink, irrespective of whether you have something ready to send at any moment or not (or whether you have something that needs to be sent at the offered rates!).
More recently, we have seen the evolution of both of these network architectures towards more flexible support for multiple service categories. Multiservice IP and Broadband ISDN, provided by ATM are both being redesigned from the ground up to cater for actual perceived multimedia application requirements.
To this end, the notion of Traffic Classes, each of which have a range of parameters (usually known as Quality of Service parameters, even though they are quantitative) have been designed. In the ITU and ATM Forum, these are called bearer service classes, and in the IP Integrated Services Internet work, they are Flow Classes.