In the United States House of Representatives, members are usually selected from congressional districts that are made out of a certain part of the state. In certain circumstances, however, the residents of the state have a representative for the entire state.
The first way this would happen is to have a representative (possibly several) that represented the state at large, in addition to representatives voted from individual districts. Various states have done this over the years, but currently none do so. The other type of at-large congressional district, and the one that is currently in use, is simply when a state has too small of a population to have more than one district. The following list of states only has one representative, picked from the state as a whole:
In addition to these normal congressional districts, the non-voting congressional delegates
from these areas are also elected at large: