The societies found in Westeros are all loosely inspired by a wide swath of Middle Ages societies, with a bit of later, Renaissance society thrown in. This is mixed with some distinctly fanciful notions to present what’s seen in the novels.
Please note that the following text is a very brief overview of life in the Seven Kingdoms. For more in-depth information, make sure to consult the Articles, which cover many roleplay-specific topics, and the Concordance at the Citadel.
Broadly speaking, the contiguous kingdoms from the North to the Reach fit within the mold of the Middle Ages as experienced in western Europe roughly in the eras between the late 13th century and the mid-15th century. The North tends to be nearer the earlier end of that scale, while the Reach tends to fall nearer the later end; this can be taken to cover variations in dress, and a general sense that some areas are “rougher” or more “refined” in matters such as social etiquette.
There is a rigidly stratified society, with noble houses commanding the greater part of the wealth and ruling over great swathes of land, merchants holding another portion of material wealth, and then smallfolk toiling away as small landholders or renting. Unlike the early Middle Ages, smallfolk are not serfs, trapped on the land they work and at the pure mercy of a lord. Circumstances can, however, vary depending on the local lord’s scruples and their ability to get away with abuses.
Slavery is forbidden by all the religious teachings popular in the mainland, and by royal decree; outside of the Iron Islands, however, slavery appears to never have been practiced in the Seven Kingdoms.
Legal matters in the Seven Kingdoms are extremely complicated by the varying prerogatives of the crown, the nobles, and the people. Conflicting precedents exist for most legal situations. For the smallfolk, their only general recourse is to appeal to their local lord for judgment. For nobles, matters may be escalated to a mutual overlord, and can go as far as the king’s court. However, Westeros has enough lawlessness in it that sometimes lords may take matters into their own hands, privately feuding and even on occasion going to war with one another.
Most of the Seven Kingdoms are rigid when it comes to gender-roles, with those roles being much as you’d expect in the traditional view of the Middle Ages, where men fought and ruled and women managed the household. Inheritance is through gender-biased primogeniture, with men preferred over women in almost all cases. Only when a woman is the only surviving direct heir can she inherit according to law, but custom shows that women can be put aside in favor of a male indirect heir due to various circumstances.
Homosexuality in the setting is seen as something worthy of ridicule and even some distaste. However, religious injunctions against homosexual relations are weak (as compared to certain periods of the Middle Ages). Those of homosexual inclination do not generally advertise it, and there may be friction within families over the matter, but it is not something that would ruin lives.
Dorne, unlike the rest of the Seven Kingdoms, has another influence besides the typical Western European Middle Ages: Moorish Spain and North Africa. The desert climate on the one hand, where a few rivers are vital for the survival of the people, certainly fits Spain. The inaccessible mountains and the long-standing conflicts on the borders between the mountain houses and the families on the other side might reflect the situation in the Pyrenees, or (as Martin has suggested) on the Welsh Marches.
Due to the influence of the Rhoynar, the Dornish are somewhat more egalitarian when it comes to gender. Men are still the warriors, but women have much more of a role in the politics of rule. Primogeniture is gender-neutral, with the eldest born inheriting regardless of gender under most circumstances. Women may hold high offices.
Dorne is also rather open-minded when it comes to sexual affairs, with less stock placed in a woman’s virginity, and even in marriage. It is not uncommon for a lord to have a publicly-acknowledged paramour, a mistress who is accorded nearly as much respect as a wife. For that matter, a lady may well have a paramour. It’s not even unknown for a lord and lady to be married, and to each also have a publicly-acknowledged paramour. Last but not least, Dorne has a relaxed attitude towards homosexuality, even more so than the rest of Westeros, and same-gender paramours are also not unknown.
The Iron Islands is strongly influenced by the Vikings of the 9th and 10th centuries. They are a sea-faring culture, with their own god and their own views that mark them as distinct. They are, in many ways, more divergent from the norm of Westeros than the Dornish are. Though they are fairly rigid when it comes to matters of gender, there is somewhat more scope for women to take on “male” roles, such as becoming captains of ships and even warriors.
The Iron Islands are relatively poor, their chief wealth being the iron they pull from their mines. This, and much other labor, is carried out by thralls. Thralls are essentially like serfs, and the Iron Islands are the sole place where this is practiced. The fact that the practice persists there suggests that the Iron Throne has shown little interest in attempting to change the culture. Also, the fact that thralls cannot be bought or sold, and that the children of thralls are considered free, may give further explanation for why this practice has gone on without being stopped. The Iron Islanders have been known to take thralls in raids, including taking women to have as concubines called “salt wives”.