What is a stem (vartalo)?

A stem is always used when you attach endings to the word. In most cases a word's basic form and a stem are the same: koulu ~ koulussa.

Sometimes a stem differs from a basic form

Some words have a stem (vartalo) which differs from the nominative.
(The partitive is, however, different and you need to memorize these three forms of words: Nominative, Partitive, Stem).

You can recognise which word type it is by looking at the end of the word in nominative.
This is how the types are named: e-sanat (words ending in e in nominative: huone~huonee-),
-sanat (words ending in nen in nominative nainen~naise-),
-sanat (words ending with i: hotelli~hotelli-, kieli~kiele-),
si-sanat (words ending with si: uusi~uude-) etc.