Lego Technic Parts Details and Usage Example - 1: Peg connectors used in a claw

Though small in stature, pegs are big in terms of utility, providing you with the ability to connect beams and other kind of parts that have holes.Pegs come in with different colors, lengths, end shapes and they are designed  for different purposes.

1. Friction or smooth pegs

Pegs can either be frictional or smooth. A smooth peg at pivot point of two beams allows free spin of the beams. A simple robotic claw shown in the following images has two grey pegs as pivot points. The claw can be controlled to open and close smoothly. However if we replace the grey pegs with black frictional ones the claw may become very clumsy and it has to be pushed hard to open and close.

If we take a closer look at the grey and black pegs which are 2 modules in length, we can see there are some tiny differences:

Black peg (friction) has a gap in the middle

The end gaps of the black peg are slightly longer than
 of the grey one 

Color codes for smooth and friction pegs:
2 modules frictional pegs are black and smooth pegs are grey.
3 modules frictional pegs are blue and smooth pegs are tan.

Other types of pegs:

left1: 4 modules peg with bushing end

left2: 2 modules peg with axel end

left3: 3 modules blue friction peg

right: 1 module smooth peg