I think that they are refering to araneomorphs vs myglamorphs ....
one of the differences is the fang structure...myglamorphs go up and down, araneomorphs are side to side.
It's way more complicated than that, but that is a start anyway!
I'd suggest going to the ATS site www.atshq.org
They have lots of free downloadable articles that can give you alot more information!
Also, most araneomorphs have only one set of book lungs - this is 1 reason T's are looked at as primitive. True spiders have 3 tarsal claws for handling silk, and more spinnerets and more silk glands that produce various silk types as opposed to T's.
T's may be primitive, but they're beautiful and far out live true spiders.
I guess we by primitive mean 'less refined' or some such..I tend to think of them as just ancient. Just like sharks and tuataras, they've managed to stay for a long time without changing their form much.