I am not familiar with PCLinuxOS, but my guess for linux-nonfb is for a non-framebuffer mode boot.
Some computers (often laptops) need framebuffer mode to display properly.
As for fail safe boot mode, it is a terminal (ie text only) mode normally for running some
administrator/maintenance stuff, especially if your systems gets screwed up!
It is often runlevel 1 but it maybe 2 or 3 (depending on distro, see /etc/inittab).
Runlevel 1 is single user mode and 2, 3, 4, & 5 are multi-user modes (distro dependent).
Each mode has its own startup script (see /etc/rc.1, /etc/rc.2, ...) that gets executed.
This way you can control what drivers/programs get loaded/executed for each runlevel.
Also see telinit for some more info.