The Stamp Tool: Understanding What it Does
The stamp tool is essentially a brush that, instead of painting a solid color, samples a specific chunk of pixels from elsewhere in the document - or even an entirely different document - and clones them upon your image. Like a regular brush, you can change the hardness (1-100%, 1 being the softest), the size, tip shape and even blending modes. However, in addition to this, the stamp tool can sample from not only the active layer, but an entire, multi-layered document.
The fact that it can easily sample from a multi-layered document can present a few problems though. Advanced users can easily take advantage of this feature, but new users might quickly find that they find their document spattered with strange effects. This is largely due to the fact that you might forget that this tool will sample adjustment layers as well, lending to some unwanted changes in color, hue, saturation, because essentially you will be applying the effect to the area twice.
Still, this is a tool worth getting to know, because you'll find it is great for airbrushing away blemishes, dirt, damage, and several other unwanted things in photos.