Reindeer moss is something of a misnomer of a name given to the common grey/green lichens of the Cladonia family. These lichen can be found spread across northern temperate areas (northern Europe, Asia, Russia and America) and also in the Arctic.
The lichen is usually grey or bluey green, but other members of the cladonia family can be more yellowy. Beloved by model railway enthusiasts, reindeer moss looks a little bit like a close knit forest close to, made up of many trunks and branches. These are ideal for creating vegetation on model train tracks, and there are several internet sites devoted to the export of specially dyed lichens from Scandinavia for this purpose. (See below)
Reindeer moss, in the right climate, can form dense, lush carpets across the ground. These carpets are delightful to walk across, as your feet can sink deeply into them and almost disappear completely as you wander. However, although very enjoyable, it is not good news for the lichen as they are very fragile and easily damaged by tramping!
The lichen has formed a vital part of the diet of many northern peoples, although mainly in the form of reindeer meat. In these more northern areas, more complex green leafed plants will not be able to survive for most of the year, and in the colder months lichens will be the only food source. The reindeer can smell the lichen through the snows, dig it up and feed on it voraciously. Lichens can survive very cold conditions, shutting down their vital systems until it is warm enough to grow once again. However, it will take a patch of lichen 50 or so years to recover after being chomped by reindeer, caribou, sheep or musk ox. To humans the lichen taste bitter and brittle, and needs to be boiled before consumption to remove any acids that have built up inside it. After boiling and drying it can be ground up and eaten with milk.
Unfortunately lichens are very susceptible to pollutants within the the air as rather than getting their sustenance from the soil and a root system, they gather all of their food from the atmosphere. This makes them an excellent coloniser species, (able to grow on rocks without needing a soil to be there first, and in time creating a soil for more complex mosses, ferns and finally grasses and plants to grow upon) but also very much at risk from pollution. After the Chernobyl incident in 1986, tens of thousands of sheep and reindeer had to be killed as they had built up unacceptable levels of radioactive particles in their bodies. This was through consuming lichens that had soaked up these elements from the atmosphere.
For pictures of reindeer moss (and also some of those dyed lichen sites if you're interested) see