A dust devil is a strong, well-formed, and relatively long-lived whirlwind, ranging from small, half a meter wide and a few meters tall, to large more than 10 meters wide and more than 1000 meters tall. The primary vertical motion is upward. Dust devils are usually harmless, but rare ones can grow large enough to threaten both people and property.
They are comparable to tornadoes in that both are a weather phenomenon of a vertically oriented rotating column of air. Most tornadoes are associated with a larger parent circulation, the mesocyclone on the back of a supercell thunderstorm. Dust devils form as a swirling updraft under sunny conditions during fair weather, rarely coming close to the intensity of a tornado.
A landspout is a slang-term coined by meteorologist Howard B. Bluestein in 1985 for a kind of tornado not associated with the mesocyclone of a thunderstorm. The Glossary of Meteorology defines a landspout as
"Colloquial expression describing tornadoes occurring with a parent cloud in its growth stage and with its vorticity originating in the boundary layer.
The parent cloud does not contain a preexisting midlevel mesocyclone. The landspout was so named because it looks like a weak Florida Keys waterspout over land."
Known officially as "dust-tube tornadoes" by the National Weather Service, they form during the growth stage of convective clouds by the ingestion and tightening of boundary layer vorticity by the cumuliform tower's updraft. Landspouts most often occur in drier areas with high-based storms and considerable low-level instability. They generally are smaller and weaker than supercellular tornadoes, though many persist in excess of 15 minutes and some have produced F3 damage. They bear an appearance and generative mechanism highly similar to that of waterspouts, usually taking the form of a translucent and highly laminar helical tube. Like waterspouts, they are also technically considered tornadoes since they are defined by an intensely rotating column of air in contact with both the surface and a cumuliform cloud. Not all landspouts are visible, and many are first sighted as debris swirling at the surface before eventually filling in with condensation and dust.
A gustnado is a specific type of short-lived, low-level cyclonic cloud that can form in a severe thunderstorm. The name is a portmanteau of "gust front tornado", as gustnadoes form due to non-tornadic cyclonic features in the downdraft from the gust, outflow, front of a strong thunderstorm, especially one which has become outflow dominated. It has little in common with tornadoes structurally in terms of vertical development, or in regard to intensity, longevity, and formative process, as classic tornadoes are associated with mesocyclones in the upflow of the storm, not the outflow.
The average gustnado lasts a few seconds to a few minutes, although there can be several generations and simultaneous swarms. Most have the winds of an F0 or F1 tornado, and are commonly mistaken for tornadoes. However, unlike tornadoes, the rotating column of air in a gustnado usually does not extend all the way to the base of the thundercloud. Gustnadoes actually have more in common with whirlwinds (which include dust devils, whirlwinds that form due to superheated surface layers and stretched vorticity, most commonly on sunny, warm days with light winds). They are not considered true tornadoes, unless they connect the surface to the ambient cloud base, by most meteorologists and are not included in tornado statistics. Sometimes referred to as spin-up tornadoes, that term more correctly describes the rare tornadic gustnado that connects the surface to the ambient clouded base, or to relatively brief tornadoes associated with a mesovortex.
The most common setting for a gustnado is on the outflow from a severe thunderstorm (58+ mph winds). The cool air in the gust front acts like a mesoscale cold front. It slices under the warm air ahead of it, creating upward motions and turbulent interactions. The friction from this interaction creates a spinning column of air, or eddy, which can create a gustnado, to get the general idea of this, picture an area of leaves swirling on a windy day, just on a much larger scale.
Photos 1& 2 are examples of a dust devil
Photos 3 & 4 a landspout
and photo 5 of a gustnado