Notes provide an ability to store additional structured or non-structured information in an entity.

Notes are displayed as events in the entity card. They can de added to: leads, contacts, and companies.
Most of the times, notes are used by widgets to create additional time-based information.
They are always displayed chronologically in the card feed, so, if you need to add time-based information, we recommend using notes.
There are several note types:

system notes (incoming/outgoing SMS, incoming/outgoing call, contact created, etc.)
notes created by the user (text and attachment notes)

There are 10 note types in Kommo that can be edited.