From BlenderWiki

3D Views and Animation

There are some important visualization features in the 3D views that can help animation.

Keyframe Visualization

When the current frame is a keyframe for the current active object, the name of this object (shown in the bottom left corner of the 3D views) turns yellow.

Ghosts

The 3D Views can display “ghosts” of animated objects (i.e. objects which have keyframes defined for some Object Ipo curves). These ghosts are outlines of that object, as it is at the corresponding keyed frames.

To disable/enable these ghosts, you can either hit K in a 3D view, or click on the Draw Key button of the Anim Settings panel (Object context and sub-context, F7, in a Buttons window).

By default, all keyframes of the object are shown. You can display only “selected” keyframes by enabling the Draw Key Sel button, in the same panel. This implies you can select keyframes, hence that you have an Ipo Curves window in Show Keys mode (see this page).

Note also that armature’s bones have much more advanced ghosts options, described in the rigging chapter.