Clashes are effects created when two players or an NPC attack collide with each other. A clash can be performed with two sword attacks colliding, or two melee attacks colliding. Clashing with swords also emits a red spark. Clashing with melee simply gives off a shock wave. (Gives the same amount of exp you would get from actually hitting your target)
Clashing is a strong training tool, since a clash can give the same or even more exp than just training by yourself. With a safe zone, where players cannot be hurt, it becomes much easier to train via clash.
Melee Clashes are just a visual effect, while Sword clashes actually have a function. When two (or four) swords collide, the clash will stop both swordsmen, freezing them in place for 1-2 seconds. This also applies to special moves, such as Lion's Strike or Raging Serpent.