The Adepta Sororitas, also known as the Sisters of Battle and formerly known as the Daughters of the Emperor, are an all-female division of the Imperial Cult's ecclesiastical organization known as the Ecclesiarchy or, more formally, as the Adeptus Ministorum. The Sisterhood's Orders Militant serve as the Ecclesiarchy's fighting arm, mercilessly rooting out corruption and heresy within humanity and every organization of the Adeptus Terra.
There is naturally some overlap between the duties of the Sisterhood and the Imperial Inquisition; for this reason, although the Inquisition and the Sisterhood remain entirely separate organizations, the Orders Militant of the Sisterhood also act as the unofficial Chamber Militant of the Inquisition's Ordo Hereticus.
The Adepta Sororitas and the Sisters of Battle are commonly regarded as the same organization, but the latter title technically refers only to the Orders Militant, the best-known part of the organization.
The Sisterhood serves as the Ministorum's only official military force because the Decree Passive laid down by the reformist Ecclesiarch Sebastian Thor held that in the wake of the Age of Apostasy, the Ecclesiarchy cannot maintain any men under arms. This was supposed to limit the power of the Ecclesiarchy. However, the Ministorum were able to circumvent this decree by using the all-female force of the Sisterhood.