A foglet is an individual unit of utility fog, a microbot that floats in the air. Each foglet would be designed to communicate and connect, as needed, with thousands or millions of other foglets.
Currently, neither utility fog nor foglets exist, although they have been well explored in science fiction. Being substantially larger and more complex -- individually, at least -- than nanobots, they are better able to act on a human scale. As opposed to goo, foglets generally do not focus on altering the surrounding environment, but rather on forming into objects.
Foglets are generally conceived of as spheres no larger than a speck of dust, each with numerous connecting arms. These arms allow them to rapidly reconfigure from diffuse clouds to macro-level structures that humans can interact with. In their most advanced form they can construct any desired object that a human might want (aside from food and drink) out of thin air, and dissolve back into the atmosphere when humans are done interacting with the object. Given sufficiently advanced technology, anything from computer terminals to buildings can simply crystallize out of the air.