What are imposters?

AFAIK, they are billboards that represent 3d objects that you just swap in place of the polygons when you don't need to render them in full 3d (ie, when they are too far away to tell the difference between something that is 3d and something that is 2d).

As MikeyO says, they are billboards to represent 3D objects, often used when the distance to the object is large enough to be indistinguishable from the true 3D object.

Imposters are slightly more advanced than normal billboards in that they often provide different images depending on the angle from which the object is viewed.

So, an imposter may have images of a tree rendered from 8 discrete angles (at 45 degree intervals) and select the appropriate one for the billboard.

Sometimes imposters are a rendered version of the mesh - created (with alpha) on the fly (render to texture) for instance, when objects are in the distance and the angle or rotation doesn't change for many frames. Then a new imposter is rendered when the amount of error (because of rotation or distance) exceeds the threshold.

They're used often to render many soldiers/people, and clouds (btw) among other things (trees, etc.) As was said, they represent a mesh, I'm just noting, sometimes (a lot of the time) they're created on the fly, as they're needed.

-Michael g.