The Sketcher workbench is a 2D used to create drawings that will be made into 3d features.
The Sketcher Workbench can be used directly or in concert with the Part Desgin Workbench. This is an important disctinction and can cause confusion.
Sketches that are within the Part Design Workbench have constraints on them that are not found when using a Sketch stand alone.
A sketch can be operated on with tools from Draft as Well as Part Design workbench.
Sketches within a body are generally on available to other sketches within the same body.