The ribosome is an important part of the translation machinery. (The others are mRNA, aminoacyl-tRNAs, and translation factors). The translation machine makes polypeptide chains according to the information encoded in the mRNA molecule
All ribosome are composed of two subunits that separate when translation terminates and reunite when an new initiation complex is formed. The eukaryotic ribosome has 40S and 60S subunits and the prokaryotic subunits are 30S and 50S. As a general rule bacterial macromolecules are smaller that eukaryotic ones and the ribosome is no exception.
Each of the subunits is made up of a combination of RNA and proteins. The ribosomal RNA molecules are named 28S, 18S etc. The "S" at the end of these names stands for a Svedberg unit. It's a measure of the size and shape of a molecule. You can tell that the 28S ribosomal RNA molecule is larger than the 18S ribosomal RNA. Similarly, the 60S subunit is the large subunit and the 40S subunit is smaller
The behavior of molecules in a liquid in a centrifuge depends on many variables including the partial specific volume of the molecule (Vr), its molecular weight (M) and the density of the solvent (ρ). The behavior is expressed as s, which is the rate of sedimentation where
s = M(1-Vr)D/RT
The values for biological molecules fall into range of "s" values between 1 and 500 × 10-13 seconds. One Svedberg unit (S) is defined as 10-13 s units.