A single-entry visa allows you to visit the country formally and reside for over to about 90 days before leaving. You are allowed to choose to spend a part of your 90 days in another Schengen country. Double-entry visa allows you to enter and exit Finland doubly.
A multiple-entry visa is valid for about 90 days and also it allows you to enter the country a number of times, i.e. as many times as you wish to. The purpose of your visit to the country along with the decision of the Finnish authorities will decide the type of visa you’ll be admitted to.