Essentially, a ventifact is any rock that has been appreciably modified by wind erosion. Usually the term is applied to smaller rocks and pebbles, but larger formations are sometimes called ventifacts too. A geologist would probably not usually use this term to refer to rock formations, only to stones and pebbles.
Wind erosion is caused by more than just wind blowing over stone -- it is caused by small particles, particularly sand particles, that are carried by the wind. This leads to some standard formations among ventifacts. Particularly, that while they are polished, they are also often pitted or pocked, with small semicircular indentations.
Pebbles and small stones in particular have a distinct form. While rocks polished by water movement tend to be flattened circles or ovals, and the stones smoothed in a glacial till tend to be irregularly shaped, ventifacts tend to form facets. As the wind blows on the stone, it smooths down a flattened surface on the top-windward side of the stone, leaving the rest of the stone untouched. Eventually, the stone is likely to be undermined as the wind erodes the ground beneath it, rotating to present another side to the wind. The result is a a number of elongated facets intersecting along 'sharp' edges (not cutting sharp, just pointed). The shape is sometimes compared to that of a brazil nut. These stones may even be mistaken for an eroded man-made artifact.
In larger rock formations, the wind erosion is much greater near the ground, as the wind can only lift the sand so far up off the ground. This can lead to impressive overhangs, mushroom-like formations, or deep pitted caves, as the lower rock is carved away from beneath the upper layers. These formations can also be caused by other processes (water erosion and weathering of a softer strata from below a harder one). Ventifacts will be polished, and often pitted, grooved, or fluted. Libya is well known for its impressive wind erosion.
Because the most extreme cases of wind erosion are found in arid regions with little vegetation, i.e., a desert, ventifacts can be taken as evidence that the area in which they were found was once a desert. Ventifacts can be produced in other conditions, of course. Deposits of gravel containing ventifacts have been found in northwestern Scotland and in New England -- not because these areas were deserts, but because the glaciers receding at the end of the last ice age left a barren, windy land with lots of sand and gravel.
The word ventifact was coined in the early 1900s. It is constructed from the Latin words venti-, meaning 'wind', and factum, meaning 'something made'. There is a good chance that it was modeled after the word 'artifact', otherwise it would probably have been called a 'ventilith' -- a 'wind stone'.