WELCOME to the site!
i'll try to answer your questions as best as i can...
--AR15: term refering to the AR (civilian M16 style) rifle, specifically the .223/5.56 civilian line
carbine: general term for a shorter-than-stadard lenght rifle build for more compactness and manuverablity while sacrificing some aspects of the full size (for the AR: 16in bbl or less)
--a midlength refers to the extended gas system over the standard carbine length, yet shorter than the rifle length system--it fits right in between the carbine and rifle length systems
its all the same system/mechanics; the middy, while maintaing the same bbl lenth as the carbine (usually 16in) supposidly makes the cycling smoother and therefore less punishing on the AR's parts--i also think that it looks better
--FF Handguard (right?): well it attaches the HG to only 1 point on the AR upper--where the bbl connects to the upper, so the front portion is "free floating"
this ensures that the as little stress (sling mounting, hand, etc) is placed on the bbl points and not vibrate it
FF is recommended if you want max accuracy; there are some drawbacks, but it all goes to use
--triggers: the stock trigger is basic; the 2 stage i believe has a 'preset' stage (where you feel the trigger about to go) and the final stage that discharges the firearm (at least i am sure thats how it works); match: i think that its another name for the 2 stage, but i am not sure--if its maybe its adjustable too...(sorry i am not that much of a trigger guy--i like the stock ones as is)
--no, the lower does not make the caliber (unless you get a bigger caliber AR, like the AR10 which uses another size reciever, but in general for the smaller caliber ARs, no it does not)
the lower is simply there as the Firearm part (must transfer through FFL or according to state laws) and as the "skeleton" to hold the parts to make the AR work
the lower can be marked any caliber, but those markings dont affect function
--i will say 'No'--the bbl/chamber/bore are integrated--one solid piece
the bbl is inserted into the upper reciever and locked on by a bbl nut
h/w, the chamber is headspaced and made for certain diamerter bullets w/in pressure limits
the standard A2/A1 buttstocks are fine-- some (like mine) have a little, just a little play, but very stable for target/bench shooting
does marronfeathers have a web site....?