Smurfs are basically mobile spore containers. The fungus (Pumilio Hyacintha Stulta, colloq. Peyo-te) spreads underground, covering many acres. It is the only known fungus which produces two-stage mushrooms: First the large stationary ones which are sometimes described as "Smurf houses". Inside these small blue sub-mushrooms grow - the Smurfs. Each "house mushroom" usually produces up to 20 Smurfs, in rare cases up to 100 Smurfs. When a Smurf reaches a certain size (3...4 cm in length, 1...5 g in weight), it disattaches from the "house mushroom" and begins to crawl around, while consuming everything edible crossing its path (slugs, berries, worms, lichen, insects, soil and some kind of animal feces). The Smurf grows, usually until it reaches 10... 20 cm (rarely up to 30 cm) in length and 50... 250 g in weight. At some point, most Smurfs stand up and begin walking on their legs, at the same time developing rudimentary speech and intelligence.
Being mobile mushrooms, Smurfs do not have a skeleton, instead a kind of hydro-skeleton (similat to that seen in many kinds of molluscs) gives their bodies rigidity. What some interpret as white caps and pants is just the natural color of respectively the upper and lower part of their bodies. Of note is the Smurf tail which contains the Spores. When stimulated sufficiently, a full tail (easily recognizable by being visibly enlarged) will release its spores explosively, catapulting several million of them into the air. When dropping onto moist soil mixed with decomposing plant matter, the spores have a good chance to grow into a new Smurf fungus (or "village" as some hopeless romantics call it).
Smurf intelligence must not be overestimated. While their somewhat humanoid shape has led some early researchers to attribute primate-level intelligence to them, they are in fact rather dumb. Most only learn in single skill for their entire existence (which can in fact last to 10 years), such as making surprise packages, baking cakes, looking pretty or just lying lazily around. This activity is usually repeated ad infinitum, irrespective of results.
Smurfs contain some powerful hallucinogens. Test subjects licking Smurfs or sucking on their tails experience psychedelic states which last up to 12 hours. Powerful optics, including ever-changing fractal shapes, surreal landscapes and fantastic creatures can be seen and conversations with god-like creatures are experienced. Some subjects do not take this altered state very well - one man who had experienced with Smurf hallucinogens was reported to run around the woods for days afterwards, constantly yelling what sonded like "JULAAAAYYYYYYYYYYYY!!!".