If you have a deep water well or are thinking of installing one, that well will probably need a submersible pump. As the name implies, this type of pump is submerged, within the well shaft and sits at or near the bottom of the well. For more shallow wells, an external well pump, that pulls the water out of the well, works fine. But for a deep well, the submersible pump, which pushes the water to the surface is much more efficient.
It is important that the pump be sized correctly for the depth of the well, the pressure you want to maintain, the flow rate you want, and the diameter of your home supply lines (e.g. 2 or 3 inch). They are rated in horsepower, common sizes are 1/2 hp, 1/3 hp, 3/4 hp and 2 hp. It is best to check with a professional plumber or at least your local guy or gal at Home Depot. They can also show you the different models and manufacturers, such as Flotec, Gould, Little Giant and Simer.
If your well is very deep, just the cost of pulling the pump out can be very high, maybe requiring a professional well driller equipment. For this reason, you must be very sure that you know what you are doing. Take your time, read and follow the installation instructions. You don’t want to pull it out and do it again. In addition, whenever you are mixing water and electricity, you need to be careful. This is one of those jobs that you really need to consider to leaving to the pros.