Eg. Someone's child runs off. The parent might shout 'come back!' They may also say 'get back here!' Which is an angrier sentence. Come back is returning to a person who says 'come back' or returning to a place such as coming back home or coming back to a party for example.
Get back, as well as the first example used, can be used to tell someone to keep away from an area. Such as a crime scene. Police will tell you to 'get back' meaning of move away from the area.
You can 'get back' something, such as letting someone borrow something and you go to the person to 'get it back'. Or the more horrible situation of someone taking something from you and you go to 'get back' whatever they took.
I hope this helps.