Edit Mode vs Object Mode
Blender's two basic ways of working
Also known as: Blender modes · Object Mode · Edit Mode · Blender Tab key · Blender sculpt mode
Definition
Object Mode and Edit Mode are the two basic Blender modes. In Object Mode you work on whole objects: move, rotate, scale, add modifiers, set materials and apply transforms. In Edit Mode you work on the mesh itself, selecting and moving vertices, edges and faces. Tab switches between them. Other modes such as Sculpt, Texture Paint, Weight Paint and Pose Mode are chosen from the mode menu.
In production
Most beginner confusion comes from being in the wrong mode: a tool that does nothing in Object Mode may work in Edit Mode, and the other way round. Check the mode menu in the viewport header and the header controls, which change with the mode. Edit Mode works only on selected objects, and with several objects selected you can edit them together with multi-object editing.