Automotive air conditioning compressors are a vital component of your auto air conditioning system. The compressor basically takes a gas refrigerant and compresses it until it's a liquid. In the liquid state, the refrigerant is able to circulate through the rest of the A/C process.
Once your refrigerant leaves the automotive air conditioning compressor, it moves to the condenser. It's there, in the condenser, where all the heat is removed from the liquid refrigerant, cooling it. That couldn't happen unless the gas had first been made into a liquid in the compressor.
All auto air conditioning parts need to be used correctly in order to function their best. For a compressor, this means you must lubricate it with the proper oil. The wrong oil could make your seals and gaskets malfunction, which would be a disaster for the entire A/C system.
When looking for a place to buy your compressor, try to shop from a place with a wide variety of brands. Your choices shouldn't be limited by the small stock of a lesser company. If you shop through a website with a wide compressor selection, you can make your choice based on quality, not just based on what's available.