The short answer is that it is usually recommended that for digital photography you expose for the highlights as they get easily blown out and for film photography it is easier to lose detail in the shadows and easier to bring back detail in the highlights and therefore you need to expose for the shadows.
The more complicated answer is that it depends on what you want to do, what you are visualizing.
We all know that a strongly overexposed part of the photo turns to white and a very underexposed part of the photo turns to black, cutting off any texture that might have been hidden in those areas. Film and sensors do have a limited ability to record light. It can only capture light between specific intensities and these intensities are defined by the film or sensor you are using, this is the so called dynamic range.
Exposing for shadows
Make sure that the histogram is not cut off on the left side. Use your camera options to overexpose, if needed. You will have a lot of detail in he dark areas of your photo but you might have blown out the highlights.
Exposing for highlights
Make sure that the histogram is not cut off on the right side. Use your camera options to underexpose, if needed. You will have detail in the bright areas, but the darker areas will be clipped to black.
Exposing for the mid-tones
There is, of course, always the alternative to ignore the highlights and the shadows and just expose for the mid-tones. The drawback is also the benefit: clipping will occur on both ends of the histogram, but not as severely as when you expose for highlights or shadows.
Conclusion
This leads nicely to a discussion of HDRI and it is maybe a good idea to revisit the section about bracketing.
