The airbag tags you're seeing on the front seats in the van you're considering are probably indicators that the van has seat mounted side airbags in the front seats, designed to protect the torso and/or chest in certain types of crashes. Some vehicles come with seat mounted side airbags in the outboard rear seats as well. If there are no airbag tags on the rear seats, then the van only has them up front.
The more common type of side airbags in both front and rear seats are the side curtain type of airbags which are designed to come down from pillars and/or upper part of the vehicle door frames like a curtain to offer head protection in some types of crashes (usually side impacts and/or rollover crashes). Vehicles with side curtain airbags usually have an airbag logo on the windshield side pillars and the door pillars between the front and back seats.
I don't believe Windstars had the seat mounted airbags available for the rear seats, and I'm not certain if the '99 model year offered the side curtain airbags as an option.
Here's a NTSHA link that explains the various types side airbags and how they work, including seat mounted ones, much better than I can:
http://www.nhtsa.dot.gov/ncap/airbags/pages/SABFAQs.htm