[Q] HTC One - looking for explanation on S-Off/S-On and Firmware topic

Hi,

I'm trying to understand once and for all few questions that I can seem to find clear answers to:
* What is the difference between s-on and s-off?
* Is there a good reason why I should attempt to s-off my device?
* Is the s-on / s-off only an HTC "issue" or is there an equivalent "issue" on other brands?
* Firmware, when should I update my firmware ? should I use custom or regular?
* What should I know about flashing a firmware before doing so, and if I must flash firmware for a new iteration of a ROM ?
* What the radio update effect? does it effect only the reception of the device and wifi/bluetooth or is it something else entirely and I misunderstood what is radio in the first place?

please keep in mind I am not a super tech guy so please try and keep the answers pretty simple.
those were a more general questions that I just always wanted to be clear on and couldn't find a satisfying answers for myself.
but I do have some more specific questions regarding my own device.

I have an HTC One, working fine with ARHD 41.0 I wanted to update to the new 51.0 ROM but there were so much talk about the new firmware
that I just wasn't sure if just flashing the ROM is good enough.
here are some specifics of my phone are so you could help me understand what should i do.

M7_UL S-ON
HBOOT - 1.44.0000
Running Android 4.4

(Don't know if any additional information is needed).

All I want to understand is when should I upgrade my firmware , radio ?
How should I proceed without damaging my phone, if I'd like the most up to date ROM on my device?

Thanks.