There are two things that I take into account when distinguishing projects from operations (like production).
The first is the relative range of uncertainty. If you were to assess, for example, the possible anticipated range of duration for a production/operations task, it might be around +/- 10%, max. Project tasks are usually more like -20% to +200%.
Another differentiating characteristic is the usual nature of queing of work in well managed project or production operations. The amount of "touch-time" on a project would typically be a much higher percentage of total time in a project than in a production-like operation. In operations, a common objective is to make sure that key constraining resources are not allowed to sit idle, so work waiting for resources is a common and relatively accepted situation. In projects, the speed of getting to the final deliverables is usually more paramount, so the preferred situation would be resources waiting for work. Admittedly the application of Lean Manufacturing brings production operations closer to the latter situation, but the basic disctinction will still probably be valid for the foreseeable future.
