What are the main differences between solid and stranded conductors?

Solid conductors are constructed of one, single piece of metal.  It is tougher than a stranded conductor, but rigid and less flexible than a stranded conductor.  Solid conductors are more likely to break if subjected to frequent flexing than stranded conductors. Stranded conductors are made of multiple small strands, which group together to make up a single conductor.  It is more flexible than a solid conductor, but less durable.

Stranded Constructions:

Bunch Stranding , Concentric Stranding, Unilay Stranding, Rope Lay Stranding

• Bunch stranding is a collection of strands twisted together in the same direction without regard to the geometric arrangement.
• Concentric stranding consists of a central wire or core surrounded by one or more layers of helically laid wires. Each layer after the first has six more wires than the preceding layer. Except in compact stranding, each layer is usually applied in a direction opposite to that of the layer under it.
• Unilay stranding is the same as true concentric; except that the lay length is the same in each layer. Normal direction of lay is left-hand.
• Rope lay stranding is a concentrically stranded conductor, each of whose component strands are they themselves stranded.  A rope stranded conductor is described by giving the number of groups laid together to form the rope along with the number of wires in each group.