Greeble – A greeble or nurnie is a small piece of detailing added to break up the surface of an object to add visual interest to a surface or object. ~ definition courtesy of Wikipidia.
Nut, Bolt, Screw, Washer, Nail, Rivet, Fastener, Crank, Buckle, Gauges, and other misc pieces. These are all the most time consuming things to make, yet can make your model/scene go from mediocre to amazing. This scene has an assortment of greeble that can easily junkify your models or be modified for a start on a more complex object.
Note: Some objects are meant to be floaters. Floaters literally float above the surface of the geometry to make the surface seem like there are bolts and such that were actually cut into the surface.
In this case this is where the “inset” pieces, and the heads of screws come in handy. This is why there are variations with no threading. So as to not eat the resources of your computer if it is not going to be seen.