Seems I have gotten ahead of myself, I have started posting about things like DTMs and TINs and I have not attempted to explain what they are. For starters, a DTM is a Digital Terrain Model and a TIN is a Triangulated Irregular Network. For the most part, I often use the terms interchangeably but there is a minute difference. Both terms represent a surface, a composition of plane segments that represent and model the shape of the earth. Both compositions contain points, connected by line segments, line segments that define edges of triangles. Adjoining triangles share both vertices and edges; the vertex of one triangle cannot lie in the interior of another triangle.
The difference lies in the orientation of the points; a DTM is most often affiliated with designed surfaces while a TIN is often associated with field collected data, hence the irregular-ness.