Taloyhtiö, housing co-op is a very common way of living in Finland. Since co-ops usually communicate only in Finnish (if at all), check out a couple of key words - in case you'll receive an info letter and don't know what it's about:
Putkiremontti, linjasaneeraus - complete renovation of plumbing, ventilation, maybe electricity and other infrastructure too. Good for the house, trouble for the folks living in the house. You usually need to move out during the project. The houses built in the 1960's, 1970's and early 1980's are now going through the putkiremontti.
Yhtiökokous - "annual general meeting". Organized usually once a year. For those who are owners in the co-op. If you own the flat together with your wife or husband and either one of you doesn't attend the AGM, make sure that the one not-attending gives a signed letter of authorization to the one who takes part.