The dust is very fine, about the same consistency as cigarette smoke. It’s a near vacuum but the winds are fast too. The winds would barely stir an autumn leaf on Earth - but they are able to lift the dust off the surface.
At their thickest the dust storms can block out 99% of the sunlight.
This shows photographs taken by Opportunity during a dust storm from sols 1205 to 1236 (one month). Each horizon view has been compressed horizontally (but not vertically). By the end of this period it reached a visual optical depth tau 4.7 which means that 99% of the sunlight was blocked. However that is for direct light. Of course the dust will also scatter a lot of light, and if you include ambient as well as direct light then the figures are not quite so extreme.
The dust particles in dust storms range from less than a micron to 50 microns in diameter.
Even so, it’s quite a challenge to explain how they form and how the dust dunes move and there are many papers on the topic. Another factor is the low Martian gravity. The larger grains can move great distances via “saltation” where the dust grain doesn’t quite get into the atmosphere, but does a series of large hops The sand dunes on Mars, rather surprisingly, move at about the same speed they do on Earth even with the very thin atmosphere.
This striking image from Mars Reconnaissance Orbiter shows quite how much sand movement you can get on Mars. In places, sand dunes as high as 200 feet (61 meters) are moving over the surface of Mars - a surprising result with its thin atmosphere (but strong winds and months long dust storms sometimes).
Meanwhile finer dust rises to many kilometers in the atmosphere. This shows dust that rises in a dust devil.
The same also happens during dust storms, and it can take months for all the dust to settle after a global dust storm.
Another puzzle is about how and why the dust storms spread to cover the whole of Mars. This happens only a few times a decade.
Then from time to time dust storms will cover the entire planet, with wind speeds of 10 to 30 meters per second (22 to 67 miles per hour) average for the faster winds during a dust storm.
Global Mars dust storm from 2001 Mars has local storms every two years, and from time to time it has larger global storms. The first global storm recorded is from 1873: the other ones reported were in 1909, 1924, 1956, 1971, 1973, 1975, 1977 (2 storms), 1982, and more recently in 1994, 2001 and 2007. So we get a global dust storm roughly every decade or so, though sometimes several per decade (five storms in the 1970s).
The global dust storms normally start in the Hellas basin in the southern spring or summer (the Martian year is two Earth years so that means every two years). But not every time for some reason.
The oval patch of dust in the leftmost of this time series of photographs is over Hellas basin.
However there is progress. This is one model from 2006 which was able to simulate global dust storms on Mars. They find that as the dry ice in the southern hemisphere evaporates in the southern spring and summer, it causes large wind systems especially on the slopes of the Hellas Basin which is close to the southern polar cap on Mars, and various other effects. The dust lifted in the storms itself affects the weather and once you have a large enough dust storm, the winds increase and more dust gets caught up until it envelopes the whole planet.
This answer uses extracts from my kindle booklet OK to Touch Mars? Europa? Enceladus? Or a Tale of Missteps? - the dust storms are particularly important for planetary protection - if microbes can get imbedded in a particle of dust, the storms could take them rather rapidly almost anywhere on Mars.
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