About clarinet reeds

The clarinet reeds are made from the cane of Arundo donax, a type of grass. Reeds may also be manufactured from synthetic and plastic materials.
Brands of reeds are:

Rico, Mitchell Lurie, Vandoren, D'Addario ...(few other brands).
All of these brands make several different varieties of reeds, in different strengths.

Reeds come in varying degrees of hardness, generally indicated on a scale from 1 (soft) through 5 (hard).  
Reed and mouthpiece characteristics work together to determine ease of playability and tone characteristics.

B clarinet reed measurements are approximately:
tip - 12 millimetres  wide
lay - 15 millimetres long (distance from the place where the reed touches the mouthpiece to the tip)
gap - 1 millimetre  (distance between the underside of the reed tip and the mouthpiece).

Theory of sound.
The ligature fastens the reed to the mouthpiece. When air is blown through the opening between the reed and the mouthpiece, the reed vibrates and produces the instrument's sound.
Reeds all look the same, but they aren't. They are produced from different qualities of cane, and cut slightly differently. Fractions of millimeters in the cut and the differences of the sound is amazing.