Coherent structures are interfaces between wave trains: they are 
time-periodic in an appropriate coordinate frame and resemble plane 
waves with possibly different wavenumbers in the farfield. We propose a 
classification of defects into four different classes which have all 
been observed experimentally. The characteristic distinguishing these 
classes is the sign of the group velocities of the wave trains to either 
side of the interface, measured relative to the speed of the interface.
We also describe stability, multiplicity, robustness, and interaction 
properties  which all depend crucially on the type of the coherent 
structures.