The materia answer will have to wait until I play through VII as "my" understanding of it is zero unless you want to waste I imagine pages of post explaining it to me back and forth. The junctioning I will take a crack at.....
So from my limited play of VIII my experience is that junctioning allows you to attach magic to boost certain stats and allow you to boost defensive and/or offensive abilities. Furthermore, correct me if im wrong here, but some junctions allow you a chance to inflict the status of the said magic junctioned either as a status ailment or added damage?
One example that comes to mind straight away is the alignment of day and weather.... sounds cosmic I know but I kid you not
Vana'diel has days based on the elements and the same for weather. I.E. if it were Firesday with single Fire weather then your damage output from Fire based magic will be increased by a percentage. In the case of double Fire weather it is increased further again. Equally if the magic you are casting is of the status ailment inflicting type and is fire based then it effects chance to land and duration, which helps in the case of sticking it to higher level mobs. Obviously thats not "junctioning" but using the properties of the environment in a "junctioning" like fashion to boost your magic ability.
For the most part though the comparison comes in the form of equipment. Adding first to what I just wrote are elemental staves as this is a good example, these directly add a pre-set and consistent 10-25% damage bonus to magic damage and chance to land ailments and duration. It is interesting that you mentioned "job classes" as I have to delve into this area to explain further. Staves in the case of the job "Summoner" directly effect the amount of MP used by the summon per "tick". "Tick" refers to MP used every 3 seconds by the summon to remain summoned.
Again "job classes" are important to this explanation. I will use the example of "Thief" as its what I played most. As you level a job you also level its inherent stats the same as for example Squalls inherent stats. In FFXI the stats are capped based on what job you are playing at the time I.E. Thief is geared towards high Agility, Dexterity, Evasion and Dagger skill. There is a whole classing system that determines what level a jobs abilities cap at but as an example:
Thief: (these are just random numbers im using)
DEX-250
AGI-250
EVA-320
Dagger Skill-300
All other stats-150-200
In the case of FFVIII and Sqaull he takes magic and junctions it to boots his inherent stats thereby increasing his damage and/or defensive ability. In the case of me and my Thief I use certain pieces of equipment to increase my stats with the same goal in mind, to increase mostly my evasion and dagger skill was my flavour. I was all about hitting and not being hit.
So you see they compare very closely they are just doing it in different ways, VIII uses magic junctions in the place of equips whereas XI is simply the reverse. With each job in XI you are simply aiming to acquire equips that boost the stats that are desirable to that job, equally in VIII you are junctioning to suit a desired task you choose a char to do.
On a final note some equips in XI will inflict status ailments or elemental damage when you are hit, The latter elemental is always el oh el but the status ailment gear is great but alot of its geared around mages in an attempt to offer some added defensive ability as they are fairly squishy jobs. And I 100% whole heartedly agree that MMORPGS eat your soul hence I dont play it any more 5yrs was enough got pretty bad there for a while but I dont regret it.