A feature class is a collection of common features with same spatial representation, such as a point, line, or polygon. For more information on feature classes, see https://pro.arcgis.com/en/pro-app/latest/help/data/geodatabases/overview/feature-class-basics.htm.

A feature layer is a group of similar geographic features mostly used for visualizing data on top of base maps. Feature layers reference feature classes for display and use in maps and scenes. They can represent different geometric data, such as point, line, polygon, etc. Feature classes can reference data from different datasources, such as geodatabase. For more information on feature layers, refer to the following documentation:

A feature layer can be a hosted layer published on ArcGIS Online or on ArcGIS Enterprise portal. It can also be published as ArcGIS Server feature service. For more information on feature layers, refer to the following documentation: