In Title 27 of the Code of Federal Regulations, you'll find all the definitions of these various items:
https://www.law.cornell.edu/cfr/text/27/479.11
It boils down like this:
1) A rifle is a rifled-barrel arm that has a shoulder stock. An SBR is such a weapon that is made to have, or is "made from a rifle", such that the barrel is less than 16" long and/or the overall length is less than 26" (with any folding or collapsing stocks fully extended).
2) A handgun is a weapon designed to fire with a hand, or hands. Its barrel must be rifled or it will fall into the category of an NFA "Any Other Weapon."
One of the sticky wickets left over from the original cobbled-apart (lol) wording of the NFA is that 26" is the limit defined as "concealable." So, the point with SBRs and SBSs was that if they were over 26" long, they were considered not a concealable weapon, and so exempt from the NFA. THis only makes sense when you understand that the original version of the NFA was supposed to prohibit handguns as well. Without that restriction, this "concealable" language makes no sense, but it's still there.
3) The fact that there are firearms that are over 26" long and which do not have any shoulder stock lead to the inclusion of the term "Other Firearm" on the form 4473 transfer paper. This is where things like pistol-only grip shotgun (type things...they aren't legally shotguns) go, as well as semi-auto belt-fed weapons with no shoulder stock, very long barreled rifled handguns that don't have stocks but still are above 26" in overall length, and also things like bare AR-15 receivers that don't have a shoulder stock (yet).
Probably the only place where the difference between a rifled long-barreled pistol and a rifled "other firearm" really matters is in the case of AR-15 based pistols and the use of vertical forward grips.
Under the NFA, any handgun that has a vertical fore-grip installed becomes an NFA "Any Other Weapon." So if your AR-15 pistol is under 26" in overall length you can't add a vertical foregrip. But, no such restriction applies to "Other Firearms." So if you build an AR-15 pistol-looking weapon that's more than 26" long, it is perfectly lawful to add a vertical foregrip to it.
Weird, huh?