A RasterBrick is a multi-layered RasterLayer. They are typically created from a multi-band file; but they can also exist entirely in memory. They are similar to a RasterStack, but processing time should be shorter. Yet they are less flexible as they can only point to a single file.
A RasterBrick can be created from RasterLayer objects, or from a file. The can also be created from SpatialPixels or SpatialGrid objects.