Expensive pipes once had stems made of amber, though this is rare now. Briar is a particularly well suited wood for pipe making for a number of reasons.
The first and most important characteristic is its natural resistance to fire.
A pipe's fundamental function is to provide a relatively safe, manipulable volume in which to incompletely combust tobacco (and/or other smokable substances) while allowing the smoke drawn from this combustion to cool sufficiently for inhalation by the smoker.
Typically this is accomplished by connecting a refractory 'bowl' to some sort of 'stem' which extends and cools the smoke mixture drawn through the combusting organic mass (see below).
The stem needs a long channel of constant position and diameter running through it for a proper draw, although filter pipes have varying diameters and can be successfully smoked even without filters or adapters.