The Content-Aware Scale is one of the major new features of the Adobe Photoshop CS4. When the aspect ratio of picture is changing during the scaling, the objects in the picture will normally lose its aspect ratio. For instance, when a 4:3 standard-definition picture is re-sized to fit to the 16:9 hi-definition screen, the people will get "fatter".  The Content-Aware Scale is a new technology that can detect the key features in the picture and protects the aspect ratio of them, while the scaling is applied only to the non-key areas.  The following example shows the difference between the conventional scale and the content-Aware scale. The height of the original photo is reduce in two different ways.

Original Photo
Normal scaling	Content-Aware Scaling

This can be very useful when the original aspect ratio of the photo ( 4:3 or 3:2 ) is not the same as the aspect ratio of the print paper or the frame and cropping is not desirable. There are always two ways to change the aspect ratio - Expanding in x-dimension has the same effect as reducing in y-dimension. However, the Content-Aware Scale algorithm behaves differently. It is thus always desirable to try both expanding or reducing in the other dimension. Manually selecting portion of the picture to protect the key area usually gives better results than the automatic default. Some examples are available here, Adobe Photoshop CS4 Content-Aware Scale.