Series 40, also known as S40, is a user interface used on Nokia feature phones from 1999 to 2014. It runs on top of a proprietary OS developed by Nokia, internally named ISA (Intelligent Software Architecture), but often just called Nokia OS. Unlike Series 60, S40 does not have its own application format, but has support for J2ME, and Flash Lite in newer versions.
The first phone to include "Series 40" was the Nokia 7110, released in 1999, which in turn was also the first phone to include a WAP browser. Despite sharing the same name, the UI of the 7110 is unrelated to other, later Series 40 phones which were released in the early 2000s with color displays.
There were also some Qualcomm based Nokia CDMA phones running Series 40 on top of REX OS, such as the Nokia 3806. The last phone to come with Series 40 was the Nokia 515, released in September of 2013.
The first version of Series 40 ran on devices with 96 × 65 or 128 × 128 displays, and had two softkeys. This UI supports primitive Java applications (up to 62 kilobytes) with MIDP 1.0, CLDC 1.0 support.
"3G Series 40"
The Nokia 6650 and 7600 were based on an unique version of 1st Edition, which is sometimes unofficially referred to as Series 45 or 3G Series 40, since they were also the first 3G Nokia phones. The 6650 and 7600 were the only 1st Edition phones to have a D-pad with a center button, allowing for three softkeys. They also had a larger, 128 × 160 display, which would become more mainstream with 2nd Edition devices.
S40 2nd Edition was released in 2003. It brought these improvements:
- Theme support using NTH files
- Support for higher resolution displays (208 × 208).
- Java ME implementation upgraded to MIDP 2.0, CLDC 1.1
The 3rd Edition of S40 was introduced in 2005 with a UI redesign, adding some major improvements:
- Support for 240 × 320 devices, but dropped 128 × 128 support
- Allows running Java apps sent via Bluetooth or Infrared
- Improved themes
- Ability to boot phone without a SIM card - this was also present on the 2610/2626 running 2nd Edition.
Feature Pack 1
- Flash Lite 1.1 introduced, meaning now the phones support SWF screensavers and Flash games
- Vector graphics support introduced
Feature Pack 2
- Bigger fonts for menus, the menus now show four items at once in list view instead of five in previous versions.
- Improved music player with Album, Artist sorting and ability to create playlists
- Flash Lite upgraded from 1.1 to 2.0
- More elements of the phone now customizable
- Slightly overhauled camera interface
As with other Nokia products, number 4 was skipped due to tetraphobia in Asian markets. 5th Edition introduced the concept of the Lite versions, which were intended for lower end devices. Lite versions do not support Flash Lite.
- An even more improved music player from S40v3 FP2 with ability to play videos, album art and changeable skins
- Slightly overhauled animations in Main Menu grid view
- Portrait images are now viewed in full-screen instead of small screen
- Java MIDP version upgraded to 2.1
Feature Pack 1
- Volume slider menu changed, now it shows an vertical bar instead of horizontal one found in previous versions.
- Later S40v5 FP1 phones received some of the features from S40v6.
The 6th edition brought these new features:
- Transition effects added throughout the menu.
- New WebKit based browser (not on 6th Edition Lite)
- Support for 320 × 480 resolution, only ever used on the 6260 slide
- Flash Lite 3
- Simplified status bar
- Clock display when pressing End/red key while the phone is locked
Feature Pack 1
Adds support for Touch and Type UI, as well as Gesture and FrameAnimator APIs, though not all FP1 phones had a touchscreen.
S40 Developer Platform
S40 Developer Platform, also known as S40 DP, is the name of S40 versions after 6th Edition, used on the Asha series, as well as later C and X series models. 
- DP 1.0 is equivalent to 6th Edition, 6th Edition Lite, or 6th Edition FP1.
- DP 1.1 is equivalent to 6th Edition or 6th Edition FP1, only used on the Asha series.
- DP 2.0 is used on Asha phones with a 240 x 400 screen resolution.
Note that this tutorial focuses specifically on games, but the procedure is similar for ringtones and other content. This requires you to own the device; it will not work on firmwares. Kahvibreak Nokia S40 Game Dumping
2nd Edition on the 6230i
3rd Edition, Feature Pack 1 on the 6233
3rd Edition, Feature Pack 1 on the 6125
3rd Edition, Feature Pack 2 on the 3500c
5th Edition on the 6267
Older 5th Edition, Feature Pack 1 on the 5310 XpressMusic
Later 5th Edition, Feature Pack 1 on the 2700c
6th Edition on the 7230