Filteraire bags are lined with melt-blown polypropelene, which is an electrostatic filter material. This 'coating' keeps the dust from escaping through the pores of the bag, but the bag tends to clog a little faster than standard non-lined bags. Filtrete bags are cloth-like. They filter very, very well and tend to keep airflow going at 100% for quite a long time.
Filtrete bags are becoming more and more the 'norm' for vac manufacturers. The new Windtunnel 2 comes with them (non Filtrete bags are also available). So does the new Constellation, but for the new "connie", it's almost a requirement since the high airflow is needed to turn the turbo brush.
My favorite Filtrete bags are those used on Miele vacuums. They are called Intensive Clean Plus, and Miele went so far as to put an aluminum foil liner in the bottom of the bag to prevent the powerful airflow from pushing dust through the cloth fibers. When the airflow comes into a bag, especially when it's angled like on a Miele, the 'stream' of air tends to 'push' the dirt (and rocks and coins and debris) against the one section of the bag opposite the intake. The foil prevents dirt from abraiding the surface of the bag and damaging it.
I guess you guys can tell I'm a Miele fan. I just wish that other companies would make Filtrete bags like Miele does, with the foil liner.
Wouldn't it be great if we could get Filtrete bags for our Electrolux model G's? Electrolux used to sell IOTA bags which went a long way towards improving the overall filtration of the machine, but stopped selling them for some reason.