The Skrulls have become the bogeyman aliens of local space because of their frequent attacks on Earth and the revelation that the Martians who attacked Earth in the early parts of the 20th century were actually Skrull subject races.
As such, a mythology has grown up around them. All Skrulls are capable of shape-shifting into any possible shape and literally spring out of the woodwork at an unsuspecting victim. They can assume superpowers at will, and are all deadly warrior males, with only a few females acting as figurehead rulers of their far flung empire.
Like all mythologies, there are truths and half truths and outright lies involved in this description, and there are a few little-known facts that would astound most casual researchers into things Skrull.
The first astonishing fact about the Skrulls is they are an offshoot of humanity. They were the victims of one of the most thorough manipulations of human germ plasm performed by the Galactics a million years ago. Most races, including the Skrulls, have no idea that there is such a connection, though the (very few) offspring of human or humanoid and Skull makes some geneticists wonder.
Rather than being a race of constantly morphing shapechangers, the Skrulls mostly have a very limited ability to change their shapes. There are some that are incredibly proficient shapechangers, but these are rare. They are trained as espionage agents and saboteurs and their abilities are legendary. Humans have come into contact with a disproportionate number of them because of the constant hostility between the two peoples.
Most Skrulls, on the other hand, have a limited ability to perhaps grow claws, or add a bit of armor to some or all of their bodies, change their skin color, or perhaps grow hair or fur and discard it at will. However, every Skrull (with a few genetically crippled exceptions) has one very profound shapechanging ability. They can shift from fully operative male to fully operative female in minutes.
The slavering warrior who conquers a planet one day may become the mother of its new settlers the next. The change is a complete one, with a full suite of hormonal changes to go with the outward sexual characteristics. For this reason, most of the day-to-day governance of the Skrull race is conducted by nurturing female Skrulls, though the war leaders are male. A Skrullís climb up the ladder of success can include rungs that are male and rungs that are female.
The warriors of the Skrull race are kept in male form through a combination of medication and meditation devised by the Skrull nation that unified their home planet. When the victory has been won and peace reigns, the warriors know how to party.
One interesting byproduct of this is that rape is actually fairly uncommon in Skrull campaigns. There have been love or political matches between Skrull and non-Skrull, and Skrull warriors are willing to pay for their pleasure, but rape is uncommon.
Many observers, especially those who have seen the so-called Super-Skrull in action, think that the Skrulls can simulate super powers as easily as they simulate the appearance of super-powered people. For the most part, this is incorrect.
Certainly exceptional Skrulls have appeared. The so-called Martian Manhunter is one such, an incredibly able psionic as well as being in the top rank of shapechangers. The Super-Skrull, on the other hand, is a very strong and pliable Skrull who has technological enhancements (including a series of satellite power boosters) that let him mimic and exceed the abilities of the Fantastic Four. Or, at least, he exceeds the abilities of the Fantastic Four at the start of their careers.
Other Skrulls have the ability to assume combat shapes that are more resistant, stronger, and faster than their normal shapes, but this is an exhausting process (akin to a human drawing on an adrenaline rush for hours at a time). It produces a Skrull who can perhaps stand up to one human mid-range super for a little while.
The Skrulls are not averse to making use of the capabilities of captive and client races. One race in particular had a profound effect on human history in the last century.
The race that first invaded Earth in 1898, then later in 1938, was a client species to the Skrulls. Known in some places as the Jovians, or Jasoomians, they came from a planet so far unknown to humans and most non-Skrull species in the Galaxy.
The Jovians are an octopoid race from a heavy-gravity world. Besides using Skrull technology with consummate expertise, they are also masters of making android bodies that they can inhabit and control, as well as creating android servitors that are strong and resilient. The Jovians also pride themselves on being able to fit into an alien society almost immediately. In this they often delude themselves, frequently succeeding in a deception simply because their victims cannot believe that they heard what they heard.
The Jovians are also said to have a slave race of their own, humanoids with an amazingly red skin and great strength and toughness. They are sometimes called Barsoomians. Both Jasoomians and Barsoomians inhabited Mars for many years before the first invasion. The ruins of their cities can be found all over Mars.