I assume you know how AFM works (If not, read question 1). There are a great variety of image types that can be displayed. Below I list the most common ones for contact and tapping modes.

CONTACT

• Height or Topography
• Deflection
• Friction
• Z sensor

TAPPING

• Height or Topography
• Amplitude
• Phase
• Z sensor

As you can see, topography images are common to both techniques. This is the type of image most commonly published. Z sensor is avialble where the microscope has a calibration sensor in the Z axis, and also represents topography. Usually the topography images are presented as a map of differently coloured pixels, with a colour bar relating the colour to a height. This is very useful, as on such an image, it's possible to estimate both lateral (xy) and height(z)measurements. However, one reason other types of image are commonly shown is that such "height maps" do not really "look like" the object in question; in other words, the appearance of a certain shape is very different to that it would have in optical (or electron microscopy). What this means, is that to the casual observer such images do not display easily the shape of the features. Ways around this include shading the image, and more commonly, creating a pseudo-3D image from the height data.

However, an alternative is to show the deflection (or amplitude) image. Because they are equivalent to a map of the slope of the sample, they often display the shape of the sample more easily. But bear in mind that the z-scale in deflection or amplitude is completley meaningless in terms of the sample structure. All it shows you is how the tip deflected as it encountered sample topography.
It is important to remember too, that the BEST images are obtained when the deflection (or amplitude) signals are minimised. This is because the deflection and amplitude images are the error signals in AFM.

The Friction, or Lateral Force images, are a map of LATERAL bending of the cantilever in contact mode. In other words, they show how the cantilever twists as it scans across the sample. This signal can be related to friction between the sample and the tip, but it also contains topogrpahic contrubutions on a non-flat sample.

The Phase images, available in tapping mode, are a map of how the phase of cantilever oscillation is affected by its interaction with the surface. The physical meaning of this signal is complicated, (see references) but in addition to topographic information, the phase can be affected by relative softness / hardness of the sample, or the chemical nature of the sample. In general, in mixed (heterogeneous)
samples, it is easy to get a contrast in the phase, but interpretation is more complicated.