First, you have to take a suite of photos of your landmark or object, following the guidelines in the Photosynth Photography Guide. Microsoft calls this "getting synthy". It's a cute marketing word, but what it means is taking hundreds of photos, and then putting Photosynth to work constructing a 3D landscape out of them.
For most of us, though, the appeal with Photosynth is exploring the synths created by other professionals--and there are many examples. From a boxer in a boxing gym to Crater Lake, Oregon to a woman reading the Origin of Species, these landscapes can be rotated around, zoomed in and out, and viewed at from practically any angle.
Photosynth requires a browser plug-in, but fortunately works with Firefox as well as Internet Explorer. It really has to be seen to be believed, and it might inspire you to take a crack at making a Synth of your own, and uploading it for the rest of the world to see.
Note: This link takes you to the vendor's site, where you can download the latest version of the software.