Harmonics are a different kettle of fish. When you have repetitive non sine waves, they are created by adding up a bunch of different sine waves. (Advanced math stuff). What happens when you connect a non-linear load like a switcher power supply, you end up creating a whole bunch non sine waves on the mains. Thru the advanced math stuff, it turns out that all of the different sine waves created are multiples of the mains frequency. It turns out that multiples of three times the fundamental mains frequency can add up and contain a significant amount of energy. These multiple of three harmonics, also know as triplens, cause problems. Anything inductive like a motor or transformer will run hotter because these harmonic currents. Phase currents in neutrals in three phase wye systems don't cancel and in fact the neutral can carry more current than any of the phases.

Typical non linear loads include most items with modern electronics like microwaves, computers, variable frequency drives, Uninterruptible power supplies, electronic lighting ballasts, etc.

Manufacturers are getting much better at producing items that play well with the power systems.

Switching power supplies are much more energy efficient but that comes at the expense of being more complex and requiring care to not create other issues.

The same item can create harmonics and have a non unity power factor at the same time.

Non linear loads are those loads that when you change the voltage by some factor, the current changes by that same factor. Resistors are linear, diodes are not. Reactive loads can be linear. The voltage changes and sometime later in the same cycle, the current will change too.

I hope this explanation helps.