When you first purchase your iPhone all you can do is call 911 until you activate with AT&T (or similar). If you are using an official Apple carrier carrier then you can activate through iTunes; otherwise you will have to perform a hack to activate your iPhone. Once activated it becomes an iPod touch with camera and bluetooth at which point you will want to proceed to jailbreak and possibly unlock it for use on other carriers.
The Baseband controls the interaction between the Phone part of the iPhone and the operating system. Whenever the iPhone needs to talk to cell towers-- whether for voice or data-- it works through the Baseband. The Baseband is responsible for making sure the sim-card is valid and for getting Signal. When you unlock your iPhone the baseband is modified to allow communication to other wireless carriers. The latest baseband version is 04.04.05_G and it corresponds to the 1.1.4 firmware.
There are at least two bootloaders in the iPhone. One is the ARM Core Bootloader and deals with the Operating System, the other is the Baseband Bootloader. Bootloaders make sure that when the iPhone boots it is doing things in the right order, and getting everything done. Currently the iPhone has shipped with two different Baseband Bootloaders (3.9 and 4.6). The 3.9BL allowed us to load a hacked baseband which matches the firmware version. Apple changed this feature in the 4.6 BL and made it require a baseband of a newer firmware version. This is why the Dev Team has released a 3.9 fakeblank BL which allows you to load a hacked baseband matching your firmware version.
DFU means Device Firmware Update. If a restore using Recovery Mode doesn't work you will want to use DFU Mode as a last resort. When placing your iPhone into DFU Mode it does not load the OS before attempting the restore. Hold the Sleep/Wake and Home buttons for 10 seconds. Just before you would normally see the Apple logo, release the Sleep/Wake button and keep hold the Home button until an iTunes detects an iPhone in recovery mode. NOTE**: This is really tricky to get the timing right it will take a few attempts. If you are in DFU Mode your screen will be black with nothing displayed.
The firmware version you have installed on the iPhone is similar to the version of the operating system on your computer. Newer firmware versions usually add new features, fix bugs, and improve stability. iPhones first shipped with 1.0.x firmware. iPhones now are shipping with 1.1.4 and version 1.2 (2.0 Beta) is available to select developers.
Installer.app is a UIKit based package manager for the iPhone. It works by downloading packages over WiFi (wireless networking) or EDGE. It supports installing, updating and uninstalling applications from multiple sources. You will use Installer.app to install a majority of your iPhone applications.
The process of gaining access to the iPhone's root filesystem, allowing third-party applications to run on the device. In general once you jailbreak you will have Installer.app, Cydia, or Icy on your SpringBoard. You can use these utilities to install most every application you would need on your iPhone.
OOB or OTB
Used to refer to the firmware version your iPhone came with "Out of the Box". ie. If you have a 1.1.2 OTB iPhone that means your iPhone came with the 1.1.2 firmware preinstalled.
If you want to do a clean restore you can put your iPhone into recovery mode to do so. Hold the Sleep/Wake and Home buttons until the iPhone reboots. Once you see the Apple logo, release the Sleep/Wake button and keep hold the Home button until an iTunes logo and the usb cable appear on screen. You can then use iTunes to restore your firmware.
A Secpack is similar to a password for the Bootloader. When passed to the bootloader, it authorizes write access to the Baseband.
SDK is short for Software Development Kit and it's basically a library of software that helps you develop applications and utilities. Apple has just released the iPhone SDK which is the set of tools needed to create applications that run on the iPhone.
The process of Unlocking allows your iPhone to use any GSM sim card. Currently Apple has limited the use of the iPhone to a select few wireless carriers. The iPhone must be unlocked before you can use any carrier of your choice.