If you select a "flat" particle - a billboard - it keeps them all aligned so that they show their flat faces towards your camera. This kind of particle is often known as a "sprite", and can be very handy - if you're not changing the camera angle radically during an animation, you can use a picture of an object as a texture on the particles, and it'll look like the emitter is producing those objects, but use far less computing power (some things work better than others, obviously). Another thing you can do is texture them with an animated texture and matching alpha for transparency (such as the ones you can make with ParticleIllusion SE from Wondertouch), to turn each particle into a fully-fledged particle animation in its own right, again with almost no computing overhead. This can produce some very sophisticated effects.