Click on the image below for a larger image, including the surrounding area. The full image is 380KB and may take a few minutes to download. This perspective view of the Swan Lake State Wildlife Area, near Portage, Wisconsin. The elevation in the DEM has been vertically exaggerated by a factor of 2. DTM is a general term for digitally created three-dimensional representations of the landscape.
TINs are similar to DEMs, but they contain points in an irregular pattern that follows the terrain.
The trigonometry involved is far too complex to summarize here, but if you want to learn more about TINs or DTMs in general, check out the links below.