The following section is based on an article in Wikipedia.
Prior to 1974 Cavan was a township in Durham County and Millbrook was a village in Cavan Township. Cavan was surveyed in 1817 and its original settlers were Irish. They were joined by United Empire Loyalist and veterans of the War of 1812--both groups were eligible for land grants. Following Canadian Confederation in 1867 the population diminished as people decided to "go west" to the newly opened Prairie Provinces of Canada.
The Township of Cavan and the Village of Millbrook became part of Peterborough County in 1974, and were amalgamated, along with North Monaghan, into one township — Cavan-Millbrook-North Monaghan — in 1998. In 2007, the township was renamed to the shorter Cavan-Monaghan.
The Archives of Ontario provide a map of the original Durham County, Ontario.
The primary source for basic documents (vital statistics, land records, wills) for people who lived in the Province of Ontario is the Archives of Ontario, 134 Ian Macdonald Blvd, Toronto, Ontario, Canada M7A 2C5.
Civil registration did not begin in the province until 1869. Before then there may be church records of baptisms and burials. For the most part these are still held by the denomination who recorded them. Copies of marriage records made pre-1869 had to be sent by individual clergymen to the registrar of the county in which the marriage took place. These marriage records are available through Ontario Archives, on micorfilm through LDS libraries, and on paid and unpaid websites, but because they were copied at the registrars' offices, they cannot be considered a primary source.
Vital Records after 1869
Birth, marriage and death registrations are not open to the public until a specific number of years after the event occurred. Births to 1914 are now available [October 2012]; dates for marriages and deaths are later. Birth and death registration was not universally carried out in the early years after its adoption. Deaths were more apt to be reported than births for several years. The more rural the area, the less likely it would be that these happenings were reported to the authorities.
Land Records and Wills
Information on how to access land records and wills is best sought on the Archives of Ontario website. An ancestor's land holding might be found on Canadian County Atlas Digital Project if he was in occupancy circa 1878.
Association for the Preservation of Ontario Land Registry Office Documents (APOLROD). A list of Land Registry Offices for all Counties of Ontario.
The original censuses are in the hands of Library and Archives Canada. All of the original census (1851-1911) images are online with the exception of that for 1861. Not all of them are indexed. Later censuses are not yet available. Census divisions were redrawn as the population increased and more land was inhabited.
E-books and Books
Some websites with more local information on Durham County
Some websites with more local information on Peterborough County