A feature layer is a grouping of similar geographic features—for example, buildings, parcels, cities, roads, and earthquake epicenters. Features can be points, lines, or polygons (areas). Feature layers are most appropriate for visualizing data on top of your basemaps. You can set properties for feature layers—such as style, transparency, visible range, refresh interval, and labels—that control how the layer appears in the map. Using a feature layer, you can view, edit, analyze, and execute queries against features and their attributes.
Hosted feature layers
Hosted feature layers are layers that have been published to your portal. The feature data in these layers is hosted by, or stored in, the data store configured with your portal's hosting server. These layers offer the most flexibility, scalability, and compatibility across the ArcGIS platform. With hosted feature layers, you can do the following:
- Edit data in the field either online or offline using Survey123 for ArcGIS or Collector for ArcGIS.
- Control editor tracking and feature layer capabilities and settings.
- Publish tile layers from feature layers to support high-demand use cases.
Hosted feature layers can be published through a variety of workflows, including ready-to-use templates.
ArcGIS Server feature layers
If you are hosting your layers on a stand-alone or federated ArcGIS Server site, you can still use these ArcGIS Server feature layers in your portal. These layers are very similar to hosted feature layers with one primary difference: the data is not stored in the hosting server's data store. Because of this, the layers have fewer configuration options available in the portal website. For example, when using an ArcGIS Server feature layer in your portal, you can't modify the editor tracking or offline capabilities of the feature layer or publish tiles from the feature layer. This guards against unwanted changes being made to the data stored in the folders and databases you registered with your ArcGIS Server site, as this data is often your system of record and is likely accessed directly by other applications. Whereas the hosted feature layer data stored and managed through the portal is not directly accessed by other applications.
If you want to work with your ArcGIS Server feature layer in your portal, you can add the layer from your ArcGIS Server site to your portal from My Content. This allows you to reference the REST endpoint (URL) of the service and use the layer in your maps and apps, while leaving the data stored in the data sources you registered with your ArcGIS Server site.
Feature collections
Items such as CSV files, shapefiles, and map notes can be added to a map as feature collections. A feature collection is a type of feature layer. Any feature collection you add can be saved as part of the map. Doing so saves the feature collection data as part of the map. Any changes you make to the feature collection—for example, by editing data—are only reflected in the map. The changes are not applied to the original CSV file, shapefile, or map note from which the feature collection was derived.
You can also save a feature collection as its own item by clicking Save Layer in the layer properties menu. The item will appear as a new feature collection item in My Content and can be shared with others and added to multiple maps. When you save a feature collection as its own item, the data is retained with the feature collection item and not stored as part of the map. Any changes you make to the feature collection item, such as modifying its data, are saved once you click Save Layer. If the feature collection item is used in other maps, the changes will be reflected there as well. If the feature collection item is deleted from My Content, it will no longer be available to others.
Map notes
You can create features on your map by adding a map notes layer. A map notes layer is a type of feature collection. With a map notes layer, you use features to symbolize something you want to show on your map, such as public access points, hiking trails, or fire perimeters. You can also add descriptive information that appears in pop-ups when the feature is clicked.
The features in a map notes layer are saved with the map so that only you, the map author, can edit and save them. These types of feature layers are useful for displaying information such as events happening within a community or an inventory of oil production facilities. This is an easy way to add a small number of features to a map, for example, the swimming pools managed by your city's parks and recreation department. It's also a way to create feature layers if you don't have portal publishing privileges.
Route layers
You can use the map viewer to get directions and create a route layer in your map. Route layers are a type of feature collection that contain four sublayers—stops, direction events, directions, and route info—each with its own properties, such as pop-ups and labels, that you can configure as desired. From the map viewer, you can save the route layer as an item in My Content and use it in other maps. Once the route layer is saved as an item, you can share it with others.
Streaming feature layers
Streaming feature layers are feature layers created from an ArcGIS Server stream service. They are useful for visualizing real-time data feeds that have high volumes of data or that have data that changes at unknown intervals. For example, a fleet of vehicles might be transmitting their location, and the current location of the vehicles needs to be continuously monitored. When you add a streaming feature layer to the map viewer, you can use streaming controls to filter the data that the service sends to the layer. Streaming feature layers can be identified by their special icon 
in My Content.