What is the difference between portable and non potable softwear?
If both are available to download, which should I prefer?

Portable software can run from a USB with no installation on the host computer.

Believe it or not, often if you install a program and afterwords copy its directory from C > Program Files to another location and then uninstall the original program, the saved directory in the other location will sometimes still run (and will also run on other PC's copied to a usb drive). So basically you made your own portable version by copying the installed directory. I do this with games all the time by installing the C drive to C > Program Files and then copying to D > Program Files > 0NoInstall (a folder I create for no installs, the 0 keeps it at the top of the folders directory tree) and uninstalling the original from C. I have to make my own shortcuts and use a nocd fix, but 95% of the time it works even without registry entries.