There are 2 ways to lay flagstone - wet lay and dry lay. Both wet and dry styles come in a variety of sizes, colours and shapes.
The flagstone is laid on a bed of dry gravel, and usually has more of a rustic look. The flagstone comes in random sizes and the joints between the stones can be filled with pea gravel, moss or even grass.
The flagstone is usually square cut and can come in different or the same size. Mortar is used to affix it, usually to a concrete slab. The joints are then filled with mortar.