# Core concepts

## Layers

A **layer** is a collection of geographic data that are used to build a map. The Foundry Map application supports a variety of layers that can be combined to form powerful geospatial visualizations:

* **Base layer:** A base layer provides the foundation of a map by rendering geographic features of the world including roads, cities, borders, place names, and more. Available base layers include a light theme, a dark theme, and satellite imagery, amongst others. Change base layers by using the selector in the **Layers** panel.

  ![Base layer selector](../../../assets/7b4e0a965697f4fb94597bc02826efa436bf05c1a413b3811fd464fe1a2dee09.webp)

You also have the option of using different types of layers as follows:

* **Object layer:** Use to leverage [geospatial data on objects](../integrate-objects/index.md) from your Ontology.
* **Link layer:** Show the relationships between objects after executing a Search Around.
* **Overlay layer:** Create high-quality visualizations using the [Map Layer Editor](../layer-editor/index.md) just once for import into one or many maps.
* **Annotation layer:** Draw shapes that highlight and provide contextual information about specific areas of your map. Read more about [creating annotations](../annotations/index.md).

## Object styling

The [style](../visualize-objects/index.md) you apply to your objects defines their appearance on a map.

## Time selection

Every map has a **selected time**, which is always within the currently selected **time window**. The time window determines the period of time for which the map loads and shows [time series](../time-series/index.md) data. [Time-based styling](../visualize-objects/index.md#opacity-styling) can use the time selection to selectively control the opacity of objects with temporal data. Read more about manipulating [time selection](../time-overview/index.md#selected-time-and-time-range).