It was actually a lot simpler than I thought and I was not quite as ignorant as I feared (both good). By general definition, tack is the equipment used on a domesticated horse. This covers saddles, bridles, reins, harnesses, bits, stirrups, and halters.

But that isn't as cut-and-dried as it sounds. There are dressage saddles, show saddles, trail saddles, English saddles, youth saddles, and Western saddles. (And probably some I missed.) Bridles, stirrups, bits, halters, and martingales have similar options. And then there are the items that complete the riding set-up, such as girths, saddle pads and blankets, stirrup leathers and pads ... and ... and ...

And ... all of those options are just to be able to ride a horse. Don't even get me started on what it takes to care for that horse. Yup, my friend is a wonder woman!