IPL: Initial program load - usually paired with Secondary Program Load (SPL), is the boot loader of your phone, much like the BIOS is the boot loader for your PC. Note: IPL/SPL are highly hardware dependent, flashing the wrong IPL/SPL is much more serious than a flashing the wrong ROM. It may be noted that, sometimes a ROM package also contains the IPL/SPL which will be flashed into your phone. Hence be extra careful what you are flashing. Note: although a ROM may also contains the IPL/SPL, `ROM` usually refers just to the OS (eg. the Windows Mobile 5, not the IPL/SPL) and the ExtRom.
The Central Processing Unit (CPU) loads the IPL Initial Programme Loader that in turn loads the SPL or Secondary Programme Loader. The SPLs main but not only function is to load the Tertiary Programme Loader TPL that de-compresses the ROM into RAM. From there the device can now be booted.
SPL: Refer to `IPL`
ROM: Read Only Memory - commonly used in the form of `ROM upgrade`. The ROM is the firmware/software, which controls everything on your phone. ROM itself usually comes in a package, with IPL/SPL, OS, Radio, and ExtRom. However, in most cases, `upgrading a ROM` usually means just the OS and the ExtRom, as these are the obvious/visible part of an ROM upgrade.
OS: Operating System - the platform software for your phone, much like the Windows Xp for your PC. Upgrading the OS is like, in PC terms upgrading from Windows 95 to Windows XP.
ExtROM: Extended ROM - is the section of the ROM which the distributor of the phone (eg O2, T-Mobile, iMate, etc) store their customization (eg Today theme, ring tones, extra software) data. In WM5, the customization data will be automatically installed at after the initial configuration (after every hardreset), just after the security section. In most cases, ExtROM can be unlocked to allow users to store/build their own customizations that will be automatically installed upon every hardreset.
CID: Carrier Id - commonly used in the form `CID-Unlocking`. If you purchase your phone (example) from Qtek (Qtek is the 'carrier'), your phone will only accept Qtek ROMs (the carrier's ROMs). If you want to flash ROMs from other carrier, you'll need to CID-unlock your phone, so that your phone will accept other ROMs. Note: In general, the phrase 'unlock your phone' usually refers to SIM-unlock, not CID unlock.
Radio: Also refered as GSM (Global System for Mobile Communications) - commonly used in the form `upgrading the Radio/GSM` - in the field of ROM upgrading. The `Radio` is essentially a ROM that controls the phone function part (as oppose to PDA function part) of your phone. Upgrading this `Radio` may have effect on your phone reception quality, battery life (optimized phone function), signal strength, etc.
GSM: Refer to `Radio`
SIM: Subscriber Identity Module - commonly used in the form `SIM unlock`. Some phones are locked to the specific telco from which you purchase your phone, eg you can't use a locked O2 phone using a Vodaphone SIM/smart card. To use SIM card from other telco, you need to SIM unlock your phone. Note: In general, the phrase 'unlock your phone' usually refers to SIM-unlock, not CID unlock.
AKU: Adaptation Kit Update - Starting with Windows Mobile 5 Microsoft began a policy of updates similar to that of the desktop windows. Rather the replacing the whole OS some functionality may be added. For example AKU 2.0 introduced push mail. These updates are distributed through the OEMs and are given to the consumer in form of ROM updates. Note that OEM may choose not to create an update with the latest AKU for their devices. Getting an AKU for your phone is like getting the SP2 (Service Pack 2) for your Windows Xp.