IPFire can be installed in various ways. There are different kinds of images that can be used. Which one to choose depends on the system you want to install IPFire on.
All installation media can be downloaded from https://downloads.ipfire.org/latest.
This is the most common installation option. You will need to burn the image on an empty CD, put it into the machine you want to install IPFire on and boot from it.
As optical discs are getting more and more out of fashion so that many systems do not have a CDROM drive any more you can also use a USB key to boot the system from. For that download the ISO image and write it to a USB key.
Some systems do not contain a CDROM drive and are not able to boot from any other media than the system disk. PC Engines's ALIX is the most prominent example of these systems. In this case you must use one of the pre-installed flash images.
There are two different images available for download. One has the serial console enabled and the graphical console disabled. This is intended to be used on systems with a serial console only (no keyboard and monitor). The other image is for system with a graphical console (“classical” desktop configuration with keyboard and monitor). To use one of these images you will need to write it to a flash card.