Latin, scira, scyra.
In most cases, the word shire was a synonym for county; it was the usage normally employed by the Domesday scribe. It is still in use with this meaning, as part of the names of many counties. It also survives less visibly in the word sheriff, the king's shire-reeve, or county official.
In origin, however, many areas other than counties were called shires, some of which survived into modern times. It has been argued that such shires were ubiquitous in early English, Scottish and Welsh history and were the building blocks from which later Hundreds and counties were often constructed. Burghshire in Domesday Book might be one such; Craven, once named Cravenshire (YKS SW,Cr1), another.
There were 31 county/shires in Great Domesday and three in Little Domesday. These counties survived until the late-twentieth century in something like their Domesday shape. For much of the intervening nine centuries the counties were the primary units of local government and the main channel of communication between Crown and people, the county court and its main officer, the sheriff, being the principal agents. The county court has been described as 'the most important institution in Anglo-Saxon England' apart from the Crown. It met biennially; and while all freemen theoretically had the right and duty to attend, lay and ecclesiastical magnates and royal officials would normally have dominated its proceedings.
The importance of counties in the structure of government explains their central role in the organisation of Domesday Book.
G.W.S. Barrow, The kingdom of the Scots: government, church and society from the eleventh to the thirteenth century (1973), documents the early history of shires and related units; James Campbell, The Anglo-Saxon state (2000), analyses the roots of English governmental structures.