Citizens of the Nordic countries (Denmark, Island, Norway and Sweden) do not need a passport or visa, only a valid identity card. The same is true for many Schengen States. Visitors from most other countries need a passport but not a visa.
Finland is a member state of the Schengen Convention. The rules for third-country nationals entering the Schengen territory have been jointly agreed by the Schengen States. If you are not sure please check at the site of the Ministry of Foreign affairs where the requirements for all countries are listed. Should you need a visa please apply for it well in advance. The handling time at the Finnish Consular Office may be up to a few months.
If you need a letter confirming your registration, acceptance of an abstract, or a formal invitation letter to support your visa application please use this form.