The Skrulls are a scientific, militaristic, and technologically advanced race of extraterrestrial reptilian humanoids that are capable of shapeshifting. The Skrulls are villains bent on invading Earth and are engaged in a prolonged war with the extraterrestrial Kree.
From the Comics
Millions of years ago, the Celestials performed genetic experiments on the reptilian ancestors of the Skrulls, resulting in three branches of Skrulls - The Prime, the Deviants and the Eternals.
Eventually the three branches warred with one another, and the Deviant branch, due to possessing the innate ability to shapeshift, were triumphant, and afterwards wiped out all members of the other two races until only two were left: the Skrull Eternal, Kly'bn, and Prime Skrull of the original non-Deviant Skrull race.
Kly'bn implored them to spare his life, as killing him would kill part of their heritage. The Deviants' leader, Sl'gur't, eventually fell in love with Kly'bn, with the two eventually becoming the gods of the Skrull pantheon. As Kly'bn could not change, Sl'gurt vowed to never keep the same shape for too long.
Taking the name Prime Skrull, the last original Skrull escaped to Earth in the 20th century and later became a member of the Underground Legion. From that point, the Skrulls began to expand their territory.
The Deviant branch later split into two more groups, the modern Skrulls and an anomaly called the Dire Wraiths, a parasitic race that could still shapeshift and had use of magic, but were not as technologically advanced as the Skrulls.
The modern Skrulls originate from the planet of Skrullos, and were originally a mercantile civilization, primarily interested in free trade and willing to share their technology with all races they deemed worthy.
When they encountered a new race, they simply transformed themselves to resemble that race. The Skrull empire that resulted from these contacts was based on free trade and mutual cooperation.