The Imperial Scouts were a group of individuals who were originally the Old Republic's exploratory and scientific organizations. When they were changed to be Imperial Scouts, they were primarily used for intelligence work. In their data archives you could find maps and records of most known worlds; everything needed to prepare for covert operations. When more information was needed, the Scouts would generally infiltrate an area disguised as merchants, pirates or, ever since the birth of the Rebel Alliance, as Rebels. Generally when they performed a mission it was the prelude to an invasion.
The Scouts generally had greater apparent freedom than most other Imperial organizations, and were often out of touch with their commanders for long periods of time, having to be trusted to serve the Empire without any signs of betrayal. Most Scouts were relatively high paid, and so highly motivated. Generally their teams would consist of between four and six operatives who were transported in a nondescript small craft, generally a stock light freighter, with no military markings. These craft were usually equipped with concealed compartments and weapons. A legend persists that the Millennium Falcon herself was originally built for this service by the Old Republic variant of the Imperial Scouts, as few civilian shipyards had the ability to create a ship with so many undetectable compartments.
Sources
"The Nuns of G'aav'aar'oon" — Challenge 43 (First appearance)