# Terminology Understanding certain key concepts is essential when customizing the backoffice. These terminologies can help you decode the purpose of code effectively: * **Repository:** An API that enables communication with a server. * **State:** A reactive container holding data, when data is changed, all its Observables will be notified. You can read more about states and observables in the [States](/umbraco-cms/customizing/foundation/states.md) article. * **Observable:** An observable is the hook for others to subscribe to the data of a State. * **Observe:** Observe describes what we do when subscribing to an Observable. * **Context-API:** The name used to serve APIs (instances/classes) for a certain context in the DOM. An API that is served via the Context-API is called a Context. You can read more about this in the [Context API](/umbraco-cms/customizing/foundation/context-api.md) article. * **Context Provider:** Enables providing a class instance as a Context API. * **Context Consumer:** Enables the consumption of a Context API. * **Controller:** An abstract term for a thing that hooks into the lifecycle of an element. Many things in our system are Controllers. * **Umbraco Controller:** Enables hosting controllers. Additionally, it provides a few shortcut methods for initializing core Umbraco Controllers. You can read more about this in the [Umbraco Controllers](/umbraco-cms/customizing/foundation/umbraco-controller.md) article. * **Controller Host:** A class that can host controllers. * **Controller Host Element:** The element that can host controllers. * **Umbraco Element:** The `UmbLitElement` or `UmbElementMixin` enables hosting controllers. Additionally, it provides a few shortcut methods for initializing core Umbraco Controllers. You can read more about this in the [Umbraco Element](/umbraco-cms/customizing/foundation/umbraco-element.md) article. Read more about how to get started with extending the backoffice in the [Backoffice Setup](/umbraco-cms/customizing/extending-overview.md) article. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://docs.umbraco.com/umbraco-cms/customizing/foundation/terminology.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.