In some ways, they are compared to mercenaries, who also go after bounties, but a different kind; they kill their targets, while Sweepers capture their targets and avoid killing. Despite their "no kill" policy, Sweepers tend to duel to the death with each other.
Jobs vary in difficulty, depending on how dangerous the criminal is, ranging from "F" to "SS". A fairly weak criminal is rated as an "F" rank job, while an extremely dangerous criminal, such as Creed Dinskenth is given an "SS" rank.
The Sweeper's Alliance was an organization created by the Chronos Number, Lin Xiao Li, in an attempt to capture and eliminate Creed. In the anime when Eve was captured by the Zero Numbers, the Sweeper's Alliance banded together to rescue her, alongside Chronos.
Informants are the peole who usually aid sweepers in tracking down their targets. They also keep them updated on important information.