The difference between a stud and a bolt is that the stud has:

A stud is characterized by having no head, which distinguishes it from a bolt. While bolts typically have a head on one end that allows for tightening with a wrench, studs are designed to be embedded into the material, leaving both ends available for nuts or other fastening mechanisms. This allows for enhanced versatility in applications where a secure connection is required on both sides of the stud. The headless design of the stud also enables it to be installed in tighter spaces where a bolt's head might not fit.

The attributes of threads and head shapes given in the other options do not accurately define the fundamental distinction between studs and bolts, which centers specifically on the presence of a head. Overall, understanding this difference is essential for selecting the appropriate fastener in various mechanical and construction applications.