An important step in many applications is obtaining the approval to proceed. This may be due to a financial outlay, risk to the business, or simply getting the validation that you are doing the right thing.
There are several ways to get approval in ServiceNow. Workflow is the most powerful. For very simple scenarios, however, Approval Rules may be sufficient (available under System Policy > Rules > Approval). These allow you to hardcode a specific user or group or use a script to determine who will be asked to make a decision if the condition matches.
When an Approval record is generated, it is always associated with a single person. That person has two choices: to approve or reject. The decision of that person is taken into account when deciding what should happen next.