Inside a standard .ipsw
There will be two files inside it. One will be called MANIFEST.PLIST and one called Firmware-some number here. The firmware file is the actual firmware for the device. Newer iPod .ipsw files also have a file named Bootloader-some number here.
Inside an advanced "iOS" .ipsw
First to mention, these .ipsw files contain a complicated "Firmware" folder which has two subfolders, all_flash and dfu. The dfu subfolder contains the iBEC (except in pre-1.1 iPhone .ipsw files) and iBSS parts of the iBoot suite (and rarely the "WTF" part as well). The all_flash subfolder surprisingly contains nothing but one subfolder, and the subfolder of the all_flash subfolder contains the LLB, the main part of the iBoot suite (which is itself named "iBoot"), and a DeviceTree, as well as any pictures the main part of the iBoot suite uses (such as the on-screen Apple logo), in an ".img3" format. Note that this ".img3" format is encrypted, thus no one can pluck any of the pictures displayed on iOS devices, or the whole iBoot suite (not even in a format readable by an ARM disassembler, forget about source code), straight from an iOS .ipsw file.
The root of the .ipsw contains three UDIF disk images. One of those disk images contains a stripped-down version of iOS which updates the system software installed on a device to the system software in the .ipsw, another one contains stripped-down version of iOS which reformats a device and installs the software from the .ipsw on that device, and the largest one contains the entire iOS system software. These UDIF disk images are encrypted as well, so no one can pluck any apps that are part of iOS, or any other parts of the iOS system software, straight from the disk images in the iOS .ipsw file.
Along with that, there is a "kernelcache" file, which contains the OS X kernel compiled in ARM machine code and the drivers for iOS devices (in a proprietary format which is also encrypted). There is also a Restore.plist contains beeswax that iTunes needs to know, and in .ipsw files with "Firmware" from iBoot-594 and later versions of the iBoot suite, there is a BuildManifest.plist containing SHSH beeswax. Everything in the .ipsw other than the Firmware folder, the three UDIF disk images, the "kernelcache", Restore.plist, and BuildManifest.plist, are files similar to "microcode" that the stripped-down version of iOS receives and "injects" into itself before updating the system software on, or re-formatting, the device.