Nokia series 40:
Series 40 software platform is the application user interface (UI) software on Nokia's broad range of mid tier mobile devices. It is the world's most widely used mobile device platform and found in hundreds of millions of devices.[1] It provides communication applications such as telephone, messaging, email client with POP3 and IMAP4 capabilities and Web browser; media applications such as camera, video recorder, music/video player and FM radio; and phonebook and other personal information management (PIM) applications such as calendar and tasks.
Series 40 is an embedded software platform that is open for software development via standard or de-facto content and application development technologies. It supports Java MIDlets, i.e. Java MIDP and CLDC technology, which provide location, communication, messaging, media, and graphics capabilities. S40 also supports Flash Lite applications.
Sample screenshot:
Note that, Series 40 uses a simpler operating system than the higher end S60 (which is based on the multi-tasking Symbian OS). Because S40 devices do not support true multi-tasking and do not have a native code API for third parties, its user interface may appear to be more responsive and faster than other Nokia platforms.
There is a huge list of mobiles based on this platform.
So we shall better categorize them further:
S40 1st Edition
S40 2nd Edition
S40 3rd Edition
S40 5th Edition
S40 6th Edition