A project has a beginning and an end. Operations keep going. Getting everyone in accounts payable to switch from using software package A to B is a project. Processing the A/P is operations. This can start to get fuzzy when you look at a big production line that is cranking out the ABC model this month and then switches over to producing the DEF model next month.
Being able to make the distinction between a project and operations is helpful because each domain has its own skill sets, areas of study, credentials, and vast bodies of knowledge. There is some cross-over, but ideally you'd like to put your Operations Research major running the production line or handling logistics and not your newly-minted PMP (and visa versa). If you're truly fortunate, you have some individuals that are equally comfortable and capable in both arenas. These folks are ideal in situations where you need to kick off an initiative as a formal project, and then transition that into daily operations down the road.
So if you're ever wondering what type of work you're doing, ask yourself if it feels more like reading a book or doing laundry.