A Hundred was an administrative area, a group of several parishes. Up to the 15th Century a Hundred had its own court to deal with minor offences. The courts continued with very few functions until 1886. The names of the hundreds were often used for Poor Law Unions and for Rural District Councils. You will find some tax records grouped by hundred in record offices, notably Land Tax. The Victoria County History, which has individual histories of the towns, parishes and hamlets of a county, is arranged by Hundred.
Hundreds of Wiltshire, with an indication of their locations, taken from a map in Lewis's Topographical Dictionary of 1835. Listed North to South, West to East. Some of the Hundreds are split into several parts. See Victoria County History of Wiltshire; the topic "A to Z index of volumes" has a list of parishes and other places for those parts of Wiltshire already covered by the Victoria County History. Also see their links to "map of areas covered" and "volumes published".