Your Spa Pack is basically the brains, or motherboard of your spa.  Its job is controlling all your spas components suck as your pump(s), heater, lights, and more.  As the main control system, having a Spa Pack that is in good condition and working properly is very important. Thousands of spas are disposed of each year because of bad control systems, when really the shell and cabinet are still in excellent shape.   You can save yourself thousands of dollars by simply replacing your spa pack (control system) rather than your entire hot tub.  If your spa gets a lot of use or is over 7 years old you can expect the control system will soon need replacing.
When shopping for a new spa pack it is important for you to know a few things about your existing one.  For example is it 120 Volt or a 240 Volt system?  What combination of items is your existing pack controlling? How many pumps do you have and how many speeds do each of those pumps have?  Do you have an ozonator or UV light?  What about lights inside the tub? How many kilowatts is your heater? Do you have a blower, circulation pump, or both? Once you know what you need to control you can determine what spa pack is best suited to your tub.
Once you have purchased a new spa pack, it is recommended to have it installed by a professional spa technician, but if you find yourself to be very handy and take the proper precaution’s for working with electrical parts then you can certainly attempt this yourself.   There is typically a  wiring diagram and instructions for installation included with the spa pack.