Polyribosomes aren't really a distinct cell structure - rather, they're a temporary arrangement of several ribosomes in close proximity. This is an efficient way of processing a highly up-regulated mRNA strand; mRNA can only be read while passing through a ribosome, and any time it spends outside of one is effectively wasted in terms of achieving maximum translation. By bringing ribosomes close together, the mRNA is able to enter a new ribosome shortly after exiting another, reducing the amount of time the mRNA spends inactive.
Cilia and flagella are entirely different structures that have little or nothing to do with polyribosomes. In essence, cilia and flagella are the same structures, but with different sizes, shapes and slightly different methods of accomplishing their functions (see source below). Both are organelles that act like propellers or antenna to interact with their environment, primarily for the purpose of motion. The filament of the cilia and flagella is similar to the gearshaft of a car, in that it plugs into a "base" within the cell that allows the filament to rotate freely. The main difference comes in the nature of their movement; flagella tend to be longer and move in a circular whiplike manner, while cilia are shorter, often more numerous, and move in a more three-dimensional and "brushing" motion.