The building blocks

• Apps / modules: each feature set (Sales, Inventory, Accounting) is a module; installing one adds models, views, menus and security rules.
• Models: database tables with business logic — res.partner (contacts), product.template, sale.order, account.move (invoices and bills).
• Views: how a model is shown — list, form, kanban, calendar, pivot, graph.
• Actions and menus tie views to the navigation.

Community vs Enterprise

Community (free, open source, LGPL) covers CRM, Sales, Purchase, Inventory, Invoicing, Website, eCommerce, POS, Project and more. Enterprise adds full Accounting, Studio, payroll for some countries, IoT and mobile apps under a per-user licence. Many Australian businesses run Community plus well-chosen OCA (Odoo Community Association) or custom modules.

Multi-company and databases

One Odoo server can host many databases; one database can hold several companies sharing products and contacts.