70S refers to the rate of sedimentation of the ribosome in a centrifuge. The "S" is a unit of measurement called the Svedberg unit. Ribosomes contain two subunits.
In prokaryotes, these subunits are 50S (large) and 30S (small). The total ribosome with both subunits is 70S in prokaryotes. (Notice that 30 + 50 = 80 and not 70. Remember this is not size and the rate of sedimentation is less when the subunits are bound then when they are separated.) Eukaryotic cells have 80S ribosomes: 60S (large) and 40S (small) subunits.