You will have to pay more. The rates vary based on the flight and your status with the carrier. For example, if you are military or business/first class, you can bring 70 LBS bags.
The standard overweight fee for normal bags (your first two) is $100 if the bag is between 50.1 LBS and 70 LBS, and $200 if the bag is between 70.1 LBS and 100 LBS. Bags over 100 LBS are not allowed. If you are bringing more than two bags and are not military or business/first class, you will face additional charges. It may be cheaper to bring a 3rd bag than to bring two bags with one or both of them being heavy.
Check the United Baggage Fee Calculator for specific information for you.
In addition, they will put a sticker on your bag saying it is heavy.