Particles

Particles are a type of Entity in Minecraft; they are graphical effects. The particles always seem to "look" at the player and will usually dissapear after a short period of time

You can reduce the amount of particles visible in the Options menu.

Types of Particles
On Minecraft 1.0.0 code you can find a variety of particles. To help modders or for curious players below is displayed the types of particles and its real name shown on the game's code. If you throw a splash potion at the ground near some cobwebs, the particles emitted from the potion that enter a cobweb will slow down. Each particles area is 8x8 pixels.