Apart from being a happy Android user with a Galaxy S based phone, I don't follow the mobile OS market very closely. I'm aware of the competition between Apple iOS and Android. I know RIM is being Blackberry phones. But when reading this article I realized I had no idea what "Symbian" was.
Apparently Symbian is the OS on Nokia and Sony Ericsson phones.
You wouldn't because it's not a brand. Nokia prefer to promote to consumers their 'series' of devices.
Symbian is the OS on the Nokia N, E, C (and iirc X) series of phones, though the traditional N series of devices from Nokia is typically considered the flagship product line and rumor has it that Nokia's Meego OS will take over there.
Nokia's legacy Series 40 OS accounts for the other product lines and the majority of their 400+ million phone shipments.
Sony Ericcson have a very limited line of Symbian devices, mostly on their legacy OS (EMP) but also Windows mobile and laterly Android.
Apparently Symbian is the OS on Nokia and Sony Ericsson phones.