No, not the things you see on the highway. A billboard is a special kind of flat surface in a 3D environment that is always oriented toward the observer. Billboarding is a technique often used to put objects into a scene that would be too costly to model out of polygons, by texturing the billboard with an image of the object. Frequently this technique is used for foliage; the illusion is good enough as long as the observer isn't right up on top of the actual billboard polygon.
So, I've been learning to use billboards, after making a remarkable discovery about an open-source game (which is providing much of the inspiration for my main current project). That game involves lots of round projectiles flying about, and instead of modeling the projectiles, they're rendered using billboards. Consider: A sphere that will look reasonably smooth is made up of hundreds of individual triangles, whereas a billboard is (typically) made up of just two. That's a savings of hundreds-less-two per projectile! Brilliant!
Anyhow, that's what I've been up to most recently. That and a bit of music composition. I've decided that this game, which is a shooting game, will be complemented best by a downtempo soundtrack. A little comedic dichotomy of violent gameplay married with soothing music.