Recruitment
The Adepta Sororitas recruit solely from the Ecclesiarchy's Schola Progenium orphanages, where future novices are supposedly raised "from infancy", genetically screened to ensure purity, indoctrinated to become an immensely loyal and zealous Imperial servant and being trained in combat as well as studying matters of religion. Only the most dedicated and capable may be selected to join the ranks of the Sisterhood, with the vast majority of the progena joining other Imperial organisations, ranging from civilian positions in the Adeptus Terra to postings in the Imperial Guard or Imperial Navy.
Organization
Unlike the various independent Chapters of the Adeptus Astartes, the Adepta Sororitas form a single vast "family" of Orders that all share a point of origin in the Shrine World of San Leor, and are all tied to either the Convent Sanctorum on Ophelia VII or the Convent Prioris on Terra. A Sister sees herself as belonging to the Adepta Sororitas as a whole as much as she sees herself belonging to her own Order, and it is not uncommon that a Sister may transfer from one into another, especially in cases where old age or injury force her to give up a more active position. Likewise, they all adhere to a singular ruleset - the "Rule of Sororitas" and the "Liber Sororitas", with only minor differences between each Order's tactics or general conduct - the latter said to reflect the personality of their founding saint.
The Sororitas do not only consist of the Orders Militant (the Battle Sisters) but also various Non-militant Orders that serve various important functions for the Ecclesiarchy or the Imperium as a whole, be it the Orders Hospitaller erecting field hospitals in war zones for the Imperial Guard, the Orders Famulous leading negotiations or arranging marriages between important noble Houses, the Orders Dialogous providing valuable translation services between various Imperial dialects or even foul xenos tongues, the Orders Sabine infiltrating newly re-discovered human colonies and paving the way for the Missionarius Galaxia, or the Orders Pronatus as the Ecclesiarchy's own scientific arm that analyzes ancient archaeotech-relics, recovers and sanctifies lost relics or even attempts to find the weaknesses of the many enemies of Mankind.
Each Order is led by a Canoness, who acts as the overall mistress of everything happening at the Order's convent. Often it is only the Canoness and her most trusted Sisters Superior who deal with any outsiders, for the Sororitas brook no disturbance from their holy tasks and value their seclusion in order to preserve their piety. The Sisters Superior act as "squad commanders" and basically adopt a new Sister as soon as she joins the Order. Various specialized positions exist, such as the Repentia Mistress leading sinners in castigation or the Mistress of Novices handling the basic training of new inductees until they are deemed ready for profession as a fully-fledged Sister.
Above the Canonesses, there are only the two Prioresses of the Convent Sanctorum and Convent Prioris, and the Abbess as the overall leader of the Adepta Sororitas and one of the candidates for a seat on the High Council of Terra.
The six major Orders Militant have several thousand (3.000-7.000 at peak times) Battle Sisters each, though there is an unknown number of minor Orders who have a hundred or so members each. A good example of the Sisterhood's overall military capabilities would be
this chart showing the Imperial forces sent to confront the 13th Black Crusade.
Lifestyle
The Sisters Militant lead what can be described as a monastic lifestyle. Similar to that of most Marines, a Sister Sororitas' life focuses on her holy duties, though the Sisterhood places a much greater emphasis on prayer in addition to traditional military training and the various tasks that the respective Order has committed itself to. In general, a Sister's life is one of constant hardship, deprivation and arduous work. The Sisters believe in suffering bringing them closer to the God-Emperor, so castigation rituals (anything from naked vigils on cold stone floors to wearing hair shirts to application of the whip) are used both for punishment as well as part of religious worship. All convents are highly isolated, and it is generally only the Canoness and her most trusted officers who have any dealings with outsiders.
Gear
Their armour and weapons are of the same quality and efficiency as the Astartes stuff, though the Marine version usually contains more gadgets, whereas the Sororitas gear is pretty much just the basic version. For example, both armies' servo suits offer the same degree of armoured protection, but the Marines are able to better interface with theirs (thanks to the Black Carapace implant, which the Sisters lack), and their suits also contain some potentially useful tools such as advanced life support and superior strength enhancement or an internal waste recycler / nutritions system. The Sororitas' Angel-pattern on the other hand is pretty much just the powered armour with some strength enhancement (enough for them to use heavy weapons solo), basic autosenses and built-in voxlink. Usually, Sororitas armour is much more ornamented, though, often sporting engraved religious scripture or affixed purity seals.
The Sisters' preferred weapons are the bolter, flamer and melta, which are also referred to as the Holy Trinity. An Order Militant's most elite warriors form the Seraphim corps; highly mobile jump-pack infantry wielding two bolt or flamer pistols and sporting a set of deadly close combat skills.
That's ... just a basic overview, mind you. On first glance, it may look as if there is not much fluff about the Sororitas out there, but over the decades quite a lot of interesting details have accumulated.
An important thing to remember is that the relationship between the Space Marines and the Sisters of Battle is somewhat complicated: On one hand, the Adeptus Astartes are truly the Emperor's blood, but on the other they are abhumans willingly welcoming psykers into their ranks and following heretical beliefs. Both Marines and Sisters respect each others' skill at arms, though, and many particularly important battle has seen them fighting side by side.
That said, there have also been times during which the Sisters Militant fought against a particular Marine Chapter, usually because said Chapter has been excommunicated and the Sisters of Battle are many Inquisitors' preferred choice for a first strike against them, as they are pretty much "the next best thing" to Space Marines but without their Chapters' loyalties towards each other. Other times such clashes are the result of a conflict between a Chapter and individual officials within the Ecclesiarchy. Known engagements include Sororitas strikes against the Sons of Malice, the Angels Vermillion , the Rainbow Warriors and the Space Wolves.
With the Imperial Guard, the relationship is much easier, for the Sisters are generally accepted as "paragons of virtue whose every action is a manifestation of the divine will of the God-Emperor of Mankind", even though by the standards of the 21st century they are fanatical zealots. The latter may manifest itself in purges directed against real or supposed heresies and genetic deviations amongst Imperial forces, rarely against Marines, but more often against Guardsmen. Generally, it can be said that Sisters and Guardsmen living door to door is an invitation for trouble, but once both armies are directed against a common enemy they seamlessly integrate into a cohesive fighting force, with the Guardsmen appreciating both the Sisters' morale-raising conduct and sermons as well as the impact they can have on a battle by themselves.
