# General Concepts _Where capitalized, words refer to their meaning in this section._ Squirro is a powerful tool to **gather, understand** and **act** on any type of data (the ‘3X model’). Every atomic piece of information that is in some way distinct in the real world (such as an individual log entry, an individual e-mail or notes from a single call) is displayed as an individual item in Squirro. We refer to these items as **documents**. Documents arrive into Squirro from **sources**. Documents, searches and other operations are organized within a **project** in Squirro. A **user** may have permissions to access one or more projects. The documents within a project are referred to as the project’s **corpus**. The corpus is held in a special database known as the **index**. A single installation of Squirro, comprising multiple projects, is called an **instance**. Each document can be assigned values for multiple **facets**. These are displayed in the left side pane of the main search screen. A **facet** is a property of a document, such as which city a particular document mentions or is relating to in some way. A document may have one or multiple **values** for a facet. So, for instance, your itinerary for your flight from London to Lagos will have both London and Lagos as distinct values for a facet.