Slamming is when you purchase a phone plan or service with one company or carrier, and your service is suddenly transferred to another company without your knowledge or permission.
Cramming is when your phone bill is wracked with charges without your knowledge. These charges can range from anything, but can be unnecessary charges that you shouldn't have to pay for.
Unauthorized Third Party Charges is when a phone company offers apps or long distance calling plans, for example, from third party companies and bill you for these services as extra charges. Make sure you are aware when you or someone who is using your phone purchases any of these third party services or apps because you will be charged for it.