Drupal use Node or not? And how to use them?
Nodes are for content, complex data managed by your website are not necessarily content. In Drupal 6 and before, using node for everything was a common practice because only nodes could benefit easily from the CCK, Views and other modules. Drupal 7 introduced the concept of entities. You can create new entity type and for one entity type your can have multiple bundles. Node is an entity type and the content types (or node types) are bundles. Using the Field API, fields are defined by bundle to attach values to entities.

So you don't need, and probably shouldn't, manage everything as nodes but instead defines your own entity types and bundles to store and manage your site data. The Entity API module provides a better API to manage entities and entity types to complete Drupal core's one. The Model Entities project provides a model entity and entity administration interface to kick-start your entity related development. For Field API usage, the Examples for Developers project provides the field_example. You can also use Features to easily export field definitions into code.