A Keeper is the term used to refer to an external address that executes a job. This can be as simplistic as calling a transaction, or as complex as requiring extensive off-chain logic. The scope of Keep3r network is not to manage these jobs themselves, but to allow contracts to register as jobs for keepers, and keepers to register themselves as available to perform jobs. It is up to the individual keeper to set up their DevOps and infrastructure and create their own rules based on what transactions they deem profitable.
A Job is the term used to refer to a smart contract that wishes an external entity to perform an action. They would like the action to be performed in "good will" and not have a malicious result. For this reason they register as a job, and keepers can then execute on their contract. Both relying on the Keep3r ecosystem to mediate in the event of a dispute.