Every mobile operating system eventually fades into obscurity, and in that sense, Symbian has done a good job of surviving in a world where its main proponent, Nokia, now makes Windows Phones. This summer, the final run of Symbian phones will make it to shelves.
While Windows Phone represents Nokia in the smartphone space, the company's main weapon for volume phone shipments now exists under the 'Asha' name, though Symbian devices still sell. With the operating system having been around in various forms as early as 1997, it has held on remarkably well.
It makes sense for the company to drop Symbian. Sales of Symbian devices have declined, and the company claims the OS to be inefficient for its future needs. They state that a Windows Phone device can be produced from scratch in 12 months, while the average Symbian handset needs 22 months of work - hardly surprising given the ancient codebase.
If you'd like a blast from the (fairly recent) past, this video of the Nokia N8 is a good chance to reacquaint with Symbian.
The history of Symbian is fairly confusing due to the number of times it has changed hands, though this article from 2004 marks when Nokia became Symbian's biggest player.