It's usually preferred to stick to one material type when plumbing a home. However, it's not uncommon to see installations where you have a mixture of copper and PEX or copper and PVC pipes. For instance, it's very common to see copper used for the pressure lines and PVC (or ABS) to be used for the drain lines or to see copper used for anything outside of the home and PVC or PEX for the interior of the home. Another reason you may transition from copper to PVC is when you have a line that is going to be used to supply water to an irrigation system. Most irrigation systems utilize PVC and polyethylene pipes to supply water to plants and sprinkler heads.
Whenever making the transition between copper and PVC (or any two different materials), it's very important to use the proper fitting. For instance, you could solder on a threaded fitting to the end of a copper pipe that can then be tightened into a threaded PVC pipe.