From Apple's silhouette iPod adverts to the Tron sequel, everybody loves light streaks and, thanks to After Effects, they're pretty easy to create using the Trapcode Particular plug-in, the demo of which is available here.
After Effects ships with CC Particle World, which is a decent enough 3D particle generator but it doesn't have the flexibility or power of Trapcode's plug-in, which is one of the best particle systems on any platform, not just After Effects. Most particle systems work in broadly the same way: an emitter in 3D space ejects particles and subjects those particles to a variety of forces, such as gravity and air turbulence. The user can control the type of particle, its velocity, rotation, spawn rate, degree of randomness and more. This often produces 'fuzzy phenomena' such as fire, smoke, meteor tails, and more abstract imagery such as light trails. The number of options in Particular can be daunting, but most controls are obvious or have real-world correlations. By experimenting, you'll soon get the hang of this amazing plug-in.