There exist different kinds of shock absorbers
for cars. There are those which are operated with gas pressure and others which work with oil pressure. The two kinds exist for the front shock absorbers
as well as for the rear ones.
Both the front and rear shock absorbers
ought to be replaced per axle to ensure the vehicle remains well balanced. If a car is unbalanced because only one of the shock absorbers has been replaced, this can lead to damage to the brake pads