Verisign owns all these certificate issuing companies: Verisign Thawte Geotrust and Rapidssl.
What is the difference between them and why is the price difference so great?
Update: Here are just a couple of differences I've come across:
GeoTrust is not chained: devices that don't properly validate the entire chain may have better luck with this one. I've seen ActiveSync work better with unchained certs than chained certs.
RapidSSL and Geotrust certs are not licensed per server, so that means you can purchase one cert for an entire webfarm.
Verisign certificates come with a free daily malware scan that alerts you if any such software is found on your site.
Only Verisign code signing certificates work with WinQual