The elements which go to make up a map can be thought of in Graph Theory terms. An area, such as an ocean, a country or conurbation, is (in Graph Theory terminology) a face. A point, such as the conceptual centre of a town, is termed a node. Linear features such as roads, railways and power lines are termed edges.
Although a road does of course actually occupy some space and its surface is just as much an area as a lake is, in practice (on all but the largest-scale maps) roads are regarded as having insignificant width; that is, they are just lines. The same applies to streams and small rivers; however, beyond a certain width (at the cartographer's discretion), it may be desirable to show the river as a face rather than an edge.
The position of a node is represented in some coordinate system; VPF allows a choice, but the DCW for example uses latitude and longitude held as degrees and decimal fractions thereof. An edge's position is defined by a series of coordinates of points along the edge; between one point and the next the edge is assumed to be sufficiently well approximated by a straight line. The position of a face is defined by its boundary, or boundaries. The VPF terminology for a boundary is ring. All faces have an outer ring and there may additionally be one or more inner rings to show "holes" in the face. For example, the face representing South Africa has an outer ring comprising its coastline and land boundaries with Namibia, Botswana, Zimbabwe, Mozambique and Swaziland. There is an inner ring which is its land boundary with Lesotho. Each ring is defined in terms of one or more edges.
In VPF a distinction is made between primitives and the features which they represent. Primitives are node, edge and face and their respective features are point, line and face. A single feature may have several corresponding primitives, or vice versa, or other more complex relationships.
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