Inside the `/config` folder, every folder will be parsed and injected into the global object `strapi.config`. Let's say, you added a folder named `credentials` with two files `stripe.json` and `paypal.json` into it. The content of these files will be accessible through `strapi.config.credentials.stripe` and `strapi.config.credentials.paypal`.
Controllers are JavaScript files which contain a set of methods called **actions** reached by the client according to the requested route. It means that every time a client requests the route, the action performs the business logic coded and sends back the response. They represent the _C_ in the _MVC_ pattern. In most cases, the controllers will contain the bulk of a project's business logic.
The controllers are defined in each `./api/**/controllers/` folders. Every JavaScript file put in these folders will be loaded as a controller. They are also available through the `strapi.controllers` and `strapi.api.**.controllers` global variables. By convention, controllers' names should be Pascal-cased, so that every word in the file (include the first one) is capitalized `User.js`, `LegalEntity.js`.
Filters are a handy way to request data according to generic parameters. It makes filtering, sorting and paginating easy and reusable (eg. `GET /users?_limit=30&name=John`).
Models are a representation of the database's structure and lifecycle. They are split into two separate files. A JavaScript file that contains the lifecycle callbacks, and a JSON one that represents the data stored in the database and their format. The models also allow you to define the relationships between them.
The models are defined in each `./api/**/models/` folder. Every JavaScript or JSON file in these folders will be loaded as a model. They are also available through the `strapi.models` and `strapi.api.**.models` global variables. Usable every where in the project, they contain the ORM model object that they are refer to. By convention, models' names should be written in lowercase.
Internationalization and localization (i18n) allows to adapt the project to different languages and serve the right content to the users. This feature is deeply integrated into the Strapi's core. It will detect the user language preference (locale) and translate the requested content using the translation files.
A plugin is like a fully independent sub-application. It has its own business logic with dedicated models, controllers, services, middlewares or hooks. It can also contain an UI integrated into the admin panel to use it easily. It allows to develop or plugin features in a project in a short time span.
The admin panel uses [Bootstrap](http://getbootstrap.com/) to be styled on top of solid conventions and reusable CSS classes. It is also using [PostCSS](https://github.com/postcss/postcss) and [PostCSS SCSS](https://github.com/postcss/postcss-scss) to keep the code maintainable.
Policies are functions which have the ability to execute specific logic on each request before it reaches the controller's action. They are mostly used for securing business logic easily.
Each route of the project can be associated to an array of policies. For example, you can create a policy named `isAdmin`, which obviously checks that the request is sent by an admin user, and use it for critical routes.
The API and plugins policies (scoped) are defined in each `./api/**/config/policies/` folders and plugins. They are respectively exposed through `strapi.api.**.config.policies` and `strapi.plugins.**.config.policies`. The global policies are defined at `./config/policies/` and accessible via `strapi.config.policies`.
Public assets are static files such as images, video, css, etc that you want to make accessible to the outside world. Every new project includes by default, a folder named `./public`.
Services are a set of reusable functions. They are particularly useful to respect the DRY (don’t repeat yourself) programming concept and to simplify [controllers](#controllers) logic.
