What are these Technic "Zapfen" ("stoppers") called in English?
The LEGO Pick A Brick store lists them as connector pegs (or, more simply, pegs) although they are more commonly known as pins in the fan community1.

The black ones have added friction for holding things together, while the light gray and beige ones are smoother and allow pieces to rotate, for example tires or beams.



1 BrickLink, Peeron