FTF files are in-development builds of WebIS software that we provide for two purposes: Public Testing, and Bug Fixes. Sometimes they are intertwined.
In allmost all cases if the FTF does not fix your issue, you can downgrade back to the last public release by simply uninstalling and reinstalling the older version. If you are here because support has sent you to get the newest version, please first check: http://www.pocketinformant.com/Forums/index.php?showforum=38 for any instructions for that particular product if any. Sometimes support sends you here because your issue is unknown and may or may not already be fixed and we can't tell simply by looking at the "fix list". Most of the time the FTFs we provide are well tested and good for public consumption.

Windows Mobile:
All of our FTF builds of our software are delivered in CAB files. For more info about the CAB file look here:
http://en.wikipedia.org/wiki/Cabinet_%28file_format%29

How to Install Using a CAB file:
To use a CAB file, either use Pocket Internet Explorer on the device to download the CAB and it will install automatically for you; or download the CAB file and copy it directly to your device and use the File Explorer to run it.

What are CAB files?
CAB files are fundamentally Windows Mobile installers. The Windows Mobile has an installation program called wceload.exe which runs whenever you open a CAB file. wceload does the actual installation and uninstallation. As such CAB files are technically just documents which contain the following info:

1) Devices it can run on
2) Registry entries to install/uninstall
3) Files to copy
4) The files themselves
5) A Setup DLL
6) Shortcuts to create

The Setup DLL has 4 entry points. One for pre-install, one for post install, and one for pre-uninstall, and one for post-uninstall.

The Setup DLL Is OUR code and primarily consists of our license acceptance in the pre-install, some extra registry setup based on user feedback in the post-install, some database erasing in the post-uninstall based on user feedback.