Solid Wood means the furniture is made of solid board, either softwood or hardwood lumber. When solid boards are used in furniture construction, they are glued together side by side along the edges. Often, a number of boards are used to make the wood more stable and reduce the chance of warping. By following a seam to the end you can always identify solid board, where you will find the "end" grain.

Veneered Plywood is used for its structural integrity and is considered part of an "all wood" product.  Layers of wood are glued together at 90-degree angles for strength.  A high quality thin layer is then applied to the top which can be finished beautifully.  At its core, the veneered plywood is real wood.  The layers turned at 90-degrees prevents the wood from warping, shrinking or splitting and makes it 45 times stronger than solid wood.

Particle board is not considered "all wood".  Particle board is made of wood scraps or sawdust held together with a resin.  It is never used in quality furniture.  If damaged, particleboard will often fracture because the material is so hard it cannot absorb a shock.