Oh! That must have been it, thank you. I'm not very familiar, I just knew the cables were cat6 and cat5. Thought it might have something to do with the shielding.

Looking over the product page, it's marked as S/FTP

Yeah don’t use grounded cables if you don’t have ground to actually ground them.

At least that’s what I was told once upon a time since EE is definitely not my forte.

Correct. More specifically, if a shield is not actually grounded, all it becomes is a re-radiating antenna for all of the EMI around it, as well as a means by which to directly transfer electrical noise from one device to another.

I never see either of these terms on sites like Newegg. How do I know which is which? Which one am I supposed to use at home?

The "U" stands for Unshielded, the "S" stands for Shielded. For home use and office use, UTP is fine. STP makes sense in "noisy" (electrical or magnetic) environments, such as factory floors with heavy machinery, clinics & hospitals with specific machinery, etc. See more details on the kind of protection the shielding offers here: https://en.wikipedia.org/wiki/Shielded_cable

Makes sense, but my point was that I've never even seen an option to buy one kind of ethernet cable or other. Is it safe to assume the $15 StarTech cables I buy from Newegg are unshielded? And does the "TP" stand for "Twisted Pair"?

Yes and yes