From BlenderWiki

[edit] Manipulators

When using the normal Transform commands (G for Grab, R for Rotation, S for Scale), they will work only parallel to the current view. The transform commands can be augmented with additional modifier keys, e.g. pressing X, Y or Z immediately after G, R, or S will constrain the transform to that global axis. Using MMB will do the same but along the nearest axis from the mouse pointer. Pressing R a second time, before pressing LMB (i.e. while the rotation command is in progress), releases the default “parallel” rotation constraint, allowing free rotation in any axis.

Read more about Axis Locking.

Manipulators provide a visual representation of the transform commands and allow movement, rotation and scaling along any axis in any mode of the 3D window.

The manipulator can also be accessed in the header of the 3D View window (⇧ ShiftLMB can select more than one manipulator at the same time):

• Hand: Enable/disable the manipulators.
• Triangle: Translation/location.
• Circle: Rotation.
• Box: Scale.
• Transform Orientation: Control the orientation of the axis for transformations.

[edit] Manipulator types

There is a separate manipulator for each Transform Command. Each can be viewed separately or in a combination.

Translate.	Rotate.
Scale.	Combination.

[edit] Other Manipulator controls

Holding down Ctrl constrains the action in certain increments (see this page for details).

Holding down ⇧ Shift when you LMB click on one of the handles, the manipulator action will be performed on the other two axes to the one you clicked on (you can let go of ⇧ Shift once you have clicked).

LMB on the white circle (largest circle around rotation manipulator) do the same as pressing R.

LMB on the grey circle (inner circle around rotation manipulator) do the same as pressing R twice (“trackball” rotation).

[edit] Manipulator Preferences

The settings of the manipulator (e.g. its size) can be found in the View & Controls section of the User Preferences window.

• Size: Diameter of Widget, in 10 pixel units.
• Handle: Size of widget Handles, as a percentage of widget radius (Size/2).
• Hot spot: Hotspot size (in pixels) for clicking Widget Handles.

[edit] Transform Orientation

The default (global) transform manipulator is very useful but in some situations it’s not (for example: scale a rotated object along the rotated axis). Luckily Blender can change the orientation of the Transform Manipulator.

Below is a list of different transform orientation types. On every image, compare the position of the manipulator axes (color axes over the object) with the global (lower left corner of the 3D window) and local (the object is an empty, so just the local axes of the object are shown) ones.

Global

The manipulator matches the global axis.

Local

The manipulator matches the object axis.

Normal

The Z axis of the manipulator will match the normal vector of the selected object. Not very useful for the empty, see the example below.

A better example using an object with face normals, selecting faces in Edit mode:

View

The manipulator will match the 3D view, Y → Up/Down, X → Left/Right, Z → Towards/Away from you.

For more precisions about the transform orientations, see this page.