What is Project Management ?

Project management is the practice of coordinating resources in a unique, temporary endeavor to achieve a set of agreed-upon objectives. 

If the activity isn't unique, then it's probably an iteration of a "process" (whether the process has been formally defined or not) or a "service" and not a "Project".

If the activity isn't temporary [i.e.: doesn't have a clear beginning and end], it's probably a "service" and not a "project".

If there aren't clear requirements/objectives/exit criteria, it's just "work" and not a "project". "We need someone to do some filing" is not a project. "We need to have the team file all outstanding 2015 documents by the end of the year" could be.

"Resources" include people-hours, equipment, software, money, facilities, etc. Basically any finite assets that can be used to attain your objectives. A skill is not a resource, but the available hours of a person with that skill are.

"Within scope", "on-time" and "on-budget" are the usual underlying constraints (known as the "triple constraint") that projects operate under, but they aren't necessarily constraints  for every project. Similarly, the Processes outlined in the PMBOK are typically addressed in any mature project management framework or methodology, but not every organization or project will require the same level of diligence and formality in each area.