Sintering occurs by diffusion of atoms through the microstructure. This diffusion is caused by a gradient of chemical potential – atoms move from an area of higher chemical potential to an area of lower chemical potential. The different paths the atoms take to get from one spot to another are the "sintering mechanisms" or "matter transport mechanisms". 
In solid state sintering, the six common mechanisms are:
# surface diffusion – diffusion of atoms along the surface of a particle
# vapor transport – evaporation of atoms which condense on a different surface
# lattice diffusion from surface – atoms from surface diffuse through lattice
# lattice diffusion from grain boundary – atom from grain boundary diffuses through lattice
# grain boundary diffusion – atoms diffuse along grain boundary
# plastic deformation – dislocation motion causes flow of matter.
Mechanisms 1–3 above are non-densifying (i.e. do not cause the pores and the overall ceramic body to shrink) but can still increase the area of the bond or "neck" between grains; they take atoms from the surface and rearrange them onto another surface or part of the same surface. Mechanisms 4–6 are densifying – atoms are moved from the bulk material or the grain boundaries to the surface of pores, thereby eliminating porosity and increasing the density of the sample.