Compressors are similar to pumps : both increase the pressure on a fluid and both can transport the fluid through a pipe. Liquid are compressible, the compressor also reduces the volume of a fluid. Some Liquids are relatively incompressible; while some can be compressed, the main action of a pump is to pressurize and transport liquids.
The primary advantage of a hermetic and semi-hermetic is that there is no route for the liquid to leak out of the system. Open compressors rely on either natural leather or synthetic rubber seals to retain the internal pressure, and these seals require a lubricant such as oil to retain their sealing properties.
Some Examples :