What is an inline six cylinder? I don't think I've ever heard of it.

The I6 is the first and the core of M-Sport division since the M88 of the M1. 6 independent throttle bodies, high oil pressure and high revving are all great characteristics of the I6 M engines from the M88 to the S54.
The M-Powered V8 and V10 are great, but the I6 is the core of the M division and to be honest I hope the new MZ4 would come with a lighter and faster version of that great engine instead of the V8.

do they preform better then the V engines?

Ford had an inline 6 that was bullet tough and had some decent pulling power when bolted up to the right tranny and rear end. Ford 300 Inline 6

Not necessarily. It'* all about bore, stroke, balance, induction, etc. that drive the performance.

As far as engines are concerned, the inline-six cylinder has been BMW'* bread and butter for a long time now. I would say the 6'* have a better track record than the 8'*. There seems to be more that can go wrong with the 8'*. Plus there are more parts to pay for, 2 valve cover gaskets to leak, 2 more spark plugs, 2 more ignition coils, etc. Some BMW 8'* have been known for oil leaks.

As far as other items are concerned the V8'* are typically associated with BMW'* bigger, more luxurious, more techno-complicated vehicles, which can also have something to do with why the 8'* seem less reliable. With the exception of the 7 series BMW'* are fairly reliable overall. BMW is ranked pretty well in consumer reports and other publications.

The I-6s are whats used in the Dodge Cummins Diesel trucks... most saabs, volvos, bmws, etc...I find it odd but not impossible that youve never seen or heard of one but you learn something new everyday

like stated previously... its all dependant on how its built and whats its built to do... i had a 200ci i-6 in my 1979 mustang, it was a torquey lil motor but lacked in hp and gearing so it wasnt fast top speed wise but itd book to 90 in no time flat and roast the tires doin it... the diesels use them due to the torque they create... most dodge motors stock only produce like 250-300 hp but 700-800 ft lbs of torque... and torque is whats needed to pull trailors easily,,, and torque wins races hehe... thats y bmws are tops in racing

btw if you wanna look at something interesting check out Porsche'* 6 cyl... its a FLAT 6... with a TwinTurbo option on some hehe

Inline or straight sixes are naturally balanced without counter-weights.
They are the smoothest engines made (So are V12s).

The V formation is used more because its' more compact I assume.