Until 1917, Finland was largely under the control of the Swedish crown. Thus, church records were the foundation of any vital records kept. They were kept in Swedish until about 1863 when Finnish was adopted as the official language and parishes gradually began switching over.
Among the church records kept in Finland, you will find the expected birth/christening, marriage, and death/burial records, but there are a few more that are not used as widely in the other Scandinavian countries.
One of these record types are the communion books of Finland. In 1686, the Swedish crown required that each minister keep track of their parishioners. This same law, which marked the beginning of the husförhörslängder (clerical surveys) in Sweden, also began the rippikirjat (kommunionböcker in Swedish).
The communion records are often more helpful than regular census records in most countries because they include more detailed information regarding a person's life. They gave the name, relationship to the head of household, birth date and place, possibly marriage and death dates and places, and where and when they moved. They also included information on the parishioner's knowledge of religion and attendance at communion. These records were kept by households living on the various farms within a town.
Another type of church record that could prove useful in Finnish research is the kirkonkirjojen kopiot (avskrifter av kyrkoböcker), or church record extracts. These records are transcriptions of the original church records done by the Finnish Genealogical Society in order to preserve the originals. They usually cover the earliest church records up to about 1850. The great thing about these is that they can often be easier to read than the original script, but the transcribers left out some information including the witnesses at christenings and other random information found in the originals.
The country of Finland did not have an official census, but there were records kept for tax purposes. Although these did not list everybody in a household, depending on a person's age, these records, too, can often help in Finnish research, and can often uncover a generation past when church records begin or fill in any gaps that may exist in the church records.
These tax records were called henkikirjat ( mantalslängder). In addition to not recording people of certain ages, there were also certain classes that were exempt from having to register: the nobility, large estate owners, soldiers, and the very poor. After 1765, however, these too were included in the rolls for statistical purposes.
Although the records of Finland are similar to those found in other countries, there are obviously some that are not used elsewhere that can be of significant help in researching Finnish ancestry.