A rock glacier (AKA a block stream) is an ill-defined beast. It is, in essence, a pile of rocks that looks or acts like a glacier:
"...a tongue-like or lobate body, usually of angular boulders, that resembles a small glacier, generally occurs in high mountainous terrain and usually has ridges, furrows, and sometimes lobes on its surface, and has a steep front at the angle of repose.
-- N. Potter Jr., 1972 Ice-cored rock glacier, Galena Creek, northern Absaroka Mountains, Wyoming. Geological Society of America Bulletin, 83, 3025-3057.
Rock glaciers commonly occur along the bases of cliffs or on the floors of cirques. They look like giant lobate lava flows made of chunks of rock, often containing boulders measuring meters in diameter. The steep sides and front of these mounds may be up to 60 meters high (~180 feet). The study of rock glaciers is only about 100 years old, the first papers published on the phenomenon appearing in the 1900-1910s. But these formations appear around the world, particularly in North America (the American Cordillera), the Alps, and central Asia.
In theory, the things called rock glaciers not only look like a glacier, but also move, (or at one point moved) like a slow, small glacier. There are various formations that might be called rock glaciers.
Perhaps the most simple cause of a rock glacier are cases in which a normal (although usually small) ice glacier carried a lot of rock - perhaps even having been buried by a rock slide. As the glacier shrunk, eventually there was only a comparatively small amount of ice underneath the rock -- enough to keep it moving along, but only just barely. Eventually the ice may melt completely, leaving only a mound of rocks in the general shape of a glacial flow.
Another possible way that such flows may be formed is as a type of mass wasting -- a slow settling of debris. When a pile of debris at the base of a cliff (a talus) is saturated with freezing and thawing water, it may start to slowly 'flow' in the manner of a glacier (normal mass-wasting movement is much slower and less visible in form, usually with a very low angle of repose). While in a true glacier movement is caused by the pressure from the weight of the ice pack compressing the ice underneath until it is plastic and can slowly flow, in this type of rock glacier it is actually the thawing of the ice that causes the flow. It can be compared to solifluction, although the rocks involved give the flow a steep angle of repose, giving the appearance of a glacial flow. In these cases flow may be aided by the glacier resting directly on top of permafrost (which the melting water cannot seep into).
It is also possible that some formations that are identified as rock glaciers are simply the remains of avalanches and landslides, and the fact that they have settled into the form of a glacier is pure coincidence.
References:
http://geology.about.com/library/bl/images/blrockglacier.htm (includes photos)
http://www.agu.org/pubs/sample_articles/cr/2002JE001864/2.shtml
http://geology.geoscienceworld.org/cgi/content/abstract/27/12/1131
http://www.agu.org/pubs/sample_articles/cr/2002JE001864/5.shtml
http://www.cosis.net/abstracts/EGU06/01856/EGU06-J-01856.pdf
Outlines of Physical Geology by Chester Longwell, Adolph Knopf, and Richard Flint. 1946.