This is a term that can encompass a variety of views on species groups and their role in the ecosystem. The terms "keystone" and "indicator" are often used to describe species in this context.
Keystone species play a pivotal role in the system in regulating populations or in modifying habitat for others. Common examples are the prairie dog, whose burrows serve other species; the beaver, who dams water and alters hydrology over a number of acres; primary excavators, e.g., the pileated woodpecker, who makes the first hole in a tree; and the largemouth bass, who affects population densities of its prey.
Indicator species are used to signal changes in conditions for other species, either as a response to stress, or in monitoring conditions as they change. The caged canary taken into the mine to warn of toxic air quality is an example of an indicator. Construction of guilds (groups of species that use selected resources in the same way) is one process for identifying indicator species. Selection of an appropriate indicator is dependent on the purposes for which it will be used.
If some form of featured species could be identified and used in project planning and management to reliably represent many other species, life would be simpler. Considering that species are distinguished from each other on the basis of differences, it is not surprising that differences among species usually hinder this representation.