These are the visual aspect of map files. They are essentially just one large image which is displayed as the "map" in-game. They also contain the heightmap data of the map, which tells TA the height of each location. It is how the game defines what is a hilltop, a slope or a steep wall.
The TNT file contains no data about what the current image is displaying, for example there is no way to tell if it is water or grass. Instead, the TNT file has a paramater called "waterheight" and that is used to calculate what is underwater or not, based upon the heightmap values. Lava is defined the same was as water, except with some special values in the map's associated OTA file. These parameters simply state if the "water" area is to be treated as something other than water. Check the OTA section for more information.
For people wanting an in-depth specification of the actual file format, you can view this document: