In Blender, the Edit mode allows more access to the shape of a single object so that it can be manipulated in order to change its shape. To enter Edit mode, use this method:
In Edit mode, the 3D View menu, Tool Shelf, and Properties all change, adding new functionality only available in Edit mode:
In Edit mode, objects are broken down into three parts:
There are many ways to think about vertices, lines, and faces. For instance, if making a kite, the vertices are the joints, the lines are the sticks, and the bits of paper are the faces. If the location of the vertices is moved, the shape of the kite will change. It's the same with a 3D object. Editing by vertices, lines, or faces will affect the rest.
The Edit mode is by default in vertex select mode, meaning any selection or transformation is applied to vertices, but it's easy to switch by locating the vertex, line, or face select buttons in the 3D View menu or by pressing Ctrl + Tab and selecting the desired option from the menu that pops up:
Try out Select mode. Select groups of vertices, edges, and faces, using circle and border select. Try moving, rotating, and scaling them and see how it effects the shape of the object. Try simple basic shapes such as cubes and circles. See how Wireframe mode affects selection. Notice how the other objects in the scene can't be accessed.
A lot of what happens in the projects will happen in Edit mode. But the individual projects will better teach you how to use it.