Before you bound headlong into applying transformations to your objects, you need to understand how coordinate systems work in 3D space. All coordinate systems in Blender are based on a grid consisting of three axes: X, Y, and Z. The X-axis typically represents side-to-side movement, whereas the Y-axis represents front-to-back movement, and the Z-axis goes from top to bottom. This grid system with axes is referred to as the Cartesian grid. The origin, or center, of this grid is at the (0,0,0) coordinate. The difference in the coordinate systems within Blender lies in the way this grid is oriented relative to a selected 3D object. Figure 3-1 shows the Coordinate System Orientation menu in the 3D View header when you left-click it.
If you're coming from another 3D program, you may find the way Blender handles coordinates a bit disorienting. Programs like 3DS Max and Maya have the Y-axis representing vertical movement and the Z-axis going from front to back. Currently, you can't change the coordinate system in Blender to match these programs, so this system is one of those things that migrating users just need to get used to.
As Figure 3-1 shows, you can choose from five orientations: View, Normal, Gimbal, Local, and Global. Working in any of these coordinate systems gives you absolute control of how your object lives in 3D space. Depending on how you'd like to transform your object, one orientation may be more appropriate than the others. Blender also gives you the ability to create custom orientations. That topic is slightly more advanced than I have room to cover in this book, but after you create a custom orientation, it also becomes available on the Coordinate System Orientation menu.
This list describes details of the five possible orientations:
All these coordinate system explanations can be (please forgive the pun) disorienting. An easy way to visualize this concept is to imagine that your body represents the Global coordinate system, and this book is a 3D object oriented in space. If you hold the book out in front of you and straighten your arms, you move the book away from you. It's moving in the positive Y direction, both globally and locally. Now, if you twist the book to the right a few degrees and do the same thing, it still moves in the positive Y direction globally. However, in its local orientation, the book is moving in both a positive Y direction and a negative X direction. To move it in just the positive local Y direction, you move the book in the direction in which its spine is pointing.
To relate this concept to the View orientation, assume that your eyes are the View axis. If you look straight ahead and move the book up and down, you're translating it along the View orientation's Y-axis. Gimbal orientation would be if you rotate the book 90 degrees toward you, rotating about its X-axis. Then it's Y and Z axes are locked together. For a clear reference, the 3D manipulator in Figure 3-2 shows the difference between the coordinate systems.
The last object you select is the active object. If you're using the Local, Gimbal, or Normal orientations and select multiple objects, the transform operations happen relative to the active object's orientation.