A contract is a predetermined legal relationship between two parties, that if one party is to be found in breach of this relationship damage will ensue.
Example: Bobby gets married to Sarah, by doing so bobby and Sarah have entered into a contract that is broken by either party damages will ensue and the contract may be voided, or parts will be unenforceable.
Rules to follow is more of a guideline of what to do to be able to continue using the service. (usually legally unenforceable)
Example: A child is running around the store, the store and the mother of the child don't need to have a contract for the store to deny the mother and son service, because they aren't following the service's rules.
This question of is a TOS a contract or rules to follow can have a major impact on how the user interacts with the company. Some company argues that agreeing to their terms is a contract and legally binding; Others argue that these are the rules to be followed when using our service.
Is the TOS a contract, or rules of service?