You can find lots of different image formats in the IPFire download section. They are all for different machines and for different ways to install IPFire. In the upcoming release IPFire 2.15, we dropped some of them and we combined some of them so that we don't need to host so much data any more.
There will be no USB installation images any more. However, the functionality is integrated in the ISO image. So, if you want to install IPFire from CD or USB media, just download the ISO image and either burn it on a CD of use cat to put it on a USB key. Then boot and install as before.
There are pre-installed images for flash drives and other system that cannot be installed with the hybrid ISO image. It comes in a version for system with a serial console and for systems with video output.
This is the only type of images for the ARM architecture.
The pre-installed Xen images for paravirtualized installations has been replaced by an install script. It is recommended to use the ISO image for fully-virtualized machines.
Our primary goal was to decrease the number (and size) of images that we need to host and we wanted to make the decision what image type to download a bit easier for the users.