Regional
Words that belong to one part of the country. Travel three hours and they change - and the locals will notice which one you used.
2 words
Mie and sie instead of mä and sä in the east. Moro in Tampere. The Turku vowels. Finland is a small country with a lot of internal variation, and dialect is a live social signal rather than a museum piece.
Every card here names the region honestly. Telling somebody a Helsinki word is "Finnish" and letting them use it in Oulu is the same error as calling a city word a national one anywhere else.
Dialect words sit beside the Helsinki layer rather than inside it. Regional words and borrowings puts the two next to each other.
Common questions
How different are Finnish dialects?
Different enough to be immediately audible and rarely enough to block comprehension. Pronouns, verb endings and a set of everyday words vary; the grammar underneath does not.
Which dialect should a learner aim for?
The one spoken where you live. The Helsinki-region spoken standard is the safest default if you are learning from abroad, since it dominates media and is understood everywhere.
One word.
Every morning.
One real Finnish word every morning - what it means, what the book form is, and when Finns actually say it.