Layers are used for all types of feature editing in ArcMap—for example, to add and modify features; update their attributes; use special tools to snap, extend, or clip feature geometries; use special data entry tools; and perform rich attribute editing.
Editing in ArcMap is done within the framework of a map document and its layers. Each feature layer references a feature class and can be used to define feature templates used in editing.
See A quick tour of editing to learn more about editing in ArcMap.