In 1920 it became obligatory for all the Finnish citizens to have a surname. If one did not have a surname, it had to be invented or a priest ordered a surname.
There is no common base for all Finnish surnames. In eastern Finland real surnames were used already in the beginning 13th century. Some of the eastern names have basis in the very old finnish pagan names, some come from Greek-Catholic names and some are western based Catholic names. The surnames began to diversify when the number of persons with the same surname started grow. This generated a lot of names where the beginning of the name is the same, but the ending has changed.
In western Finland the common way was to use firstname + name of the house as an additional name. Especially in written sources the name of the house is often written after the first name, but this does not mean that this name would have been used as surnames are used nowadays. The patronymic form first name + father's name + (daughter/son) was commonly used.
In addition to this east-west separation of names there are names attached to the social class and professions. Notably soldiers and priests have their typical surnames. For example after 1690 when a man began his military service he was commonly given a Swedish, German or Latin based names which he used during his time in the military.