Odoo is continually improved, several times a day in fact, and we call these revisions.

You can see our work at https://runbot.odoo.com/

Periodically, we bundle groups of revisions, and release a new version of Odoo:

• annually for major releases like 14, 15, 16 etc
• every few months for point releases like 15.1, 15.2, 15.3 etc.  

A change to a new RELEASE requires an UPGRADE. 

A change to a new REVISION of the same release requires an UPDATE.

UPGRADE - move from Odoo 15.1 to Odoo 15.2 (or Odoo 16).

UPDATE - move from Odoo 15 - source installed March - to Odoo 15 - source as of July.

Updating (an Odoo installation)

Refers to the process of obtaining the latest revision of the source code for your current Odoo Edition. For example, updating your Odoo Enterprise 15.0 to the latest revision. This does not directly cause any change to the contents of your Odoo database, and can be undone by reinstalling the previous revision of the source code.

Upgrading (an Odoo database)

Refers to a complex data processing operation where the structure and contents of your database is permanently altered to make it compatible with a new release of Odoo. This operation is irreversible and typically accomplished via Odoo’s database upgrade service, when you decide to switch to a newer release of Odoo. Historically, this process has also been known as a “migration” because it involves moving data around inside the database, even though the database may end up at the same physical location after the upgrade.

https://odoo-community.org/openupgrade