Blender is made of a hierarchy of Datablocks connected to Objects. The Object holds the transform info (location, rotation and scale) of each Datablock. This transform info is applied at the Object's center point (orange dot). The Datablock basically describes what the Object looks like. If you enter Edit Mode and move points, edges or faces of a cube Object, you have affected its Mesh Datablock. If you move, rotate or scale the cube in Object Mode, you have affected it on its Object level.
it will be helpful to look at the Outliner while you experiment with datablocks. Expand the hierarchy of the Cube one level so you can see the "Cube" datablock nested beneath the "Cube" Object.
Be sure to also locate the Properties panel, just beneath the Outliner by default. With the cube selected, click on the icon with the triangle (Object Data). Don't worry about anything below the line (Normals section, etc.); for now, only be concerned with the cube datablock name in the field at the top of the Properites field (Figure 1).
This is where Blender is initally confusing with its default names for Objects and Datablocks; you see "Cube" on both and you probably wonder what the difference is. You're about to see right now.
Try this or something similar: change the name of the datablock to "bar" (Figure 2) and hit the Tab key to go into Edit Mode. Hit "S" to scale it in all directions fairly small, finalizing with the LMB. Then hit "S" again, followed by "X" to constrain it in the x-direction and scale it in the x-direction a ways until it looks like a bar. To avoid initial confusion, make sure to scale the cube in Edit Mode, not Object Mode (Figure 3). Tab back into Object mode and move this object away from the origin with the transform handles.
Hit Shift + A, Add Mesh, Cone (Figure 4). Look in the Outliner for the cone's Object name to see it is called "Mesh." Rename "Mesh" to "Cone" (Figure 5).
Expand its hierarchy to see that its datablock is also named "Mesh." With the Cone still selected, tab into Edit Mode and hit "R", "Y", 180, <Enter> to rotate the cone upside-down. Tab back into Object Mode. Rename the datablock name from "Mesh" to "funnel" by LMB-clicking its name in the Outliner or the Properties panel (Figure 6).
Now, with the cone still selected, locate the triangle icon to the left of the datablock name field in the Properties panel. Click this icon to reveal a drop-down menu. Notice that "bar" and "funnel" are both in this list (Figure 7). These are the available mesh datablocks in your scene.
Select "bar" from this menu to see the cone change into the bar (Figure 8). This is because the cone is now using the "bar" mesh. More importantly, look in the Outliner to see that the name nested beneath "Cone" is now "bar" also. Tab into Edit Mode to see that both object are editable. RMB-select on a vertex and move it... both objects deform because it is the same mesh (Figure 9). Undo this change, then tab back into Object Mode.
Also, look at the Properties panel, in the datablock name field. There is now a "2" to the right of this field. This indicates how many Objects are using this mesh datablock (Figure 10). In our scene, the Cube Object is using the bar mesh, and the Cone Object is also using the bar mesh.
However, they are still separate Objects, and the easiest way to tell is by location (one of the transforms). The Cone Object that now looks like a bar, still holds the same transform information that it did before; only what it looked like changed. To further prove this point, hit "R" and rotate the object, then hit "S" and scale the object (Figure 11).
You have now modified the Cone on its Object level. It still looks like the bar (in the same proportions), only moved, rotated and scaled. The other bar remains the same, because it is linked to an independent Object ("Cube") that holds unique transform info.
Another key factor to remember is that the transforms occur at the Object center point. Therefore, if you swap out a mesh, the new mesh will put its center point in the same location as the replaced mesh.
It's important to note that in this example, both the Cube and Cone objects were Meshes. You can only swap out datablocks with the same type of datablock. For example, select the camera and look in the datablock drop-down menu; nothing else will be listed, as you cannot make the camera a mesh (Figure 12).
If you examine the Add menu (Shift + A), you will see the orange Object icons to the left. In general, you may consider each of these icons as a family of compatible datablocks. As we saw with meshes, any Object that has the triangle icon can swap out meshes with each other. This will be the case whether or not it is a cube, cone, cylinder, sphere, plane, monkey, etc. because they will all have that triangular icon in the Outliner and Properties panel.
But realize that there are several other datablock categories such as materials, textures, etc.
Almost every Object has a datablock associated with it. An exception is the Empty. An Empty holds transform info but doesn't contain geometry or have materials or textures associated with it, so that concept should make sense.
As you begin duplicating your objects, it will become more apparent (and sometimes confusing) of the power of Blender's hierarchy.