A menu is always shown at the top of the screen . This is the Sales menu in Odoo Enterprise:

Note that users may see different options depending upon their authorization (and the menu itself may change depending upon Settings).

The first icon on the left hand side is a shortcut to the list of apps

The central section contains all the menu options for the current application

You can edit this section from Settings / Menu Items (in Developer Mode), or Odoo Studio has a simple “drag and drop” interface.

The right hand section is standard across Odoo:

This contains:

1. Debug menu (if in Developer mode)
2. Reminders
3. Messages
4. Company widget
5. Odoo Studio icon (if installed)
6. User name / image and admin menu 

1. Debug menu

This is only available if developer mode is enabled.

4. Company widget

This is different in Odoo 12 and Odoo 13.

In Odoo 12 and earlier you can select one company. In Odoo 13 you can select multiple companies and choose one as the current company.

6. Admin menu

Several of these options are fairly obvious

1. Documentation is simply a link to Odoo user documentation
2. Support is a link to report problems
3. Shortcuts shows you some of the shortcut key combinations

4. Preferences

This allows users to change their password, timezone, language and other preferences.

6. Log Out

The last option lets you log out and select another database (if there are multiple databases on this Odoo instance)