Shrouders are, as their name suggests, the inhabitants of "shrouds", artificially created zones of space so tightly warped that the inhabitants are, effectively, sealed off from the rest of the universe. The shrouds are effectively impenetrable from the outside (although they can be deactivated from inside).
The Shrouders began as a race of bird-like creatures known as the Amarantin on the planet Resurgam. One group of Amarantin, the Banished, achieved space flight and inadvertently triggered one of the Inhibitors' devices for detecting intelligent, space-faring life. Unable to escape the Inhibitors, the Banished created the Shrouds and retreated into them to hide until the Inhibitors (already declining in ability) were no longer active.
Other members of the Revelation Space universe are unaware of the real nature of the Shrouders. In order to determine if Inhibitors were still active and that it was safe to leave the Shroud, one group of the Banished embedded a series of false messages into the Shroud that suggested that the Shrouders were a species that had gathered treasures and hidden them inside the Shroud. Lured by these messages, anyone who penetrated further into the Shroud would have semi-sentient programs and directives embedded by the Banished that would cause the intruders to trigger the Inhibitor device near the Banished home planet. If the Inhibitors responded to the triggering device then the Banished would know that the Inhibitors were still functioning. The program manifests itself in the novels as the "Sun Stealer" entity. It is capable of hacking virtually any human computer system and controlling starships. Eventually Ilia Volyova is able to defeat it with the Melding Plague, but not before it was able to activate the Inhibitors.
A second, dissenting group within the Banished planted a separate set of programs and directives to prevent the first group's plan from succeeding, believing that triggering the Inhibitors could lead to the discovery of the Banished hidden in the Shroud. It is implied that this agent manifests itself as the Mademoiselle, an entity with similar abilities which was able to penetrate even the Conjoiners' computer security systems.