Thinset mortar is a mixture of Portland cement and a clean, fine-grain, sand that often contains a water-absorbent material to adjust the setting speed or other modifiers such as polymer additives. The name describes how the mortar is applied. Most thinset mortars can be used to lay either thin beds or medium beds for tile. Thin beds are roughly from 1/8 inch to 1/4 inch or a bit more thick. Medium beds are up to about 1/2 inch thick. The mortar bed thickness depends on the roughness and level of the surface; thicker beds are more forgiving.
Thinset is used to bond ceramic tile, porcelain tile, or natural stone tile to a surface. It is applied in a thin layer whose purpose is simply to cement the tile to the surface, which might be a floor, a table or counter, a wall or a ceiling among other things. To get a uniform layer of that thickness, the thinset is applied with a trowel that has square notches that are 3/8 of an inch (10 cm) wide. The troweling leaves long, square, parallel ridges of mortar on the substrate. The final mortar bedis formed when the tiles are pressed into place.
Thinset is fast, easy and cheap to use because a minimum amount of mortar is required. Thinset mortar will not work well if the surface you want to tile is not flat, because the finished tile will have about the same surface curvature as the substrate. To level off depressions and prominences, or to achieve a purposefully curved area such as for a floor drain, you may have to lay down a preparatory medium or thick layer of mortar. When that layer sets, you can then use thinset to cement the tile to it in the usual way.
You can buy what is called latex thinset, which is modified with various combinations of a huge number of polymer additives, many of which are not actually latex at all. The clever manufacturers of thinset mortar choose the polymers so as to give the mortar different characteristics, including flexibility, resistance to freezing, and waterproofing.
Now you know what's under that ceramic tile when you see it. To finish the tiling job, the gaps between the individual tiles are filled with grout, which is a bit different from mortar The grout lines are often sealed with a special liquid to make them waterproof and resistant to staining.