US(UNC/UNF) Thread and Metric Thread 

Metric Thread
The European Standard M Thread – ISO 724:1993 “ISO general-purpose metric screw threads — Basic dimensions” (DIN 13) is recognizable by its cylindrical outer and inner diameter in millimeters.

There are three kinds of ISO metric threads:
Coarse
Fine
Extra fine

The thread angle is 60°. The thread depth is 0.614 x pitch.
US Thread- UNC / UNF
Thread series cover designations of diameter/pitch combinations that are measured by the number of threads per inch (TPI) applied to a single diameter.

Coarse Thread Series (UNC) is the most common designation for general application bolts and nuts. Coarse thread is beneficial, because they are less likely to cross thread, more tolerant in adverse conditions and facilitate quick assembly.

Fine Thread Series (UNF) is commonly used in precision applications. Because of the larger tensile stress areas, they have high tension strength. However, a longer engagement is required for fine thread applications than for coarse series threads to prevent stripping.

US(UNC/UNF) and Metric Thread Pitch Charts