Impasto means laying paint on so thickly that the mark of the tool stays visible on the surface. The paint is no longer just a carrier of colour: it becomes a material that stands out from the canvas and casts its own small shadow.
This is the main consequence of a thick layer. A few millimetres of relief are enough to make the picture look one way in the morning and another under a lamp in the evening, and the texture comes alive most when light falls from the side. It is worth checking where the light comes from before choosing a wall for such a painting.
A thick layer describes well the things that stand out in nature too: flower heads, tufts of grass, bark, points of light across a meadow. I usually keep it for the end and for a few places only. A canvas covered in impasto throughout loses its hierarchy and the eye has nowhere to settle.
A thick layer dries from the top and stays soft under the skin for a long time, so the painting should not be wrapped or varnished too early. It also has to go over a fatter layer, never over a lean underpainting, or it will crack in time. Transport is safer flat, because the highest ridges of paint rub off easily.
A strongly textured painting rarely needs an elaborate frame — I wrote about that in how to choose a frame for an oil painting. The oils themselves are in the paintings gallery.